After some more experiments with the Pixie kits I am suspending the Pixie for a while.
Versions of the Pixie modified for transmit only, 80m version and 630m version with modified inductors and capacitors in the frequency dependent circuits, and the components specific to the receive mode have been omitted. For example, the diode and switch for RX oscillator offset has been removed. Further, a variable capacitor ("Polyvaricon") has been connected from the crystal to ground, in place of the diode offset "capacitor")
80m:
The oscillator does oscillate in the "RX mode", and the frequency range is about 3500 - 3600kHz. When keying, however the oscillator frequency still changes frequency, sometimes in an unpredictable way (sometimes up, sometimes down).
I suspect 2 reasons:
- the oscillator voltage is not stabilised
- the load of the oscillator changes when switching to "TX" mode.
Further, the output of the transmitter with the ceramic resonator in the circuit is too low to register on the SWR/PWR meter. Not exactly a resounding success. This could be due to a lower Q of the ceramic resonators, resulting in a lower oscillator output.
630m:
I could not register any output from the oscillator. There can be several reasons:
- the oscillator may be more loaded by the buffer stage
- the capacitive load of the oscillator capacitors may be too high
- the frequency determining capacitors may have a low Q
- the Q of the ceramic resonator may be too low for solid oscillator loop gain
- the capacitor ratio in the feedback loop may result in a loop gain below 1.
My conclusion is that the basic design of the Pixie is too simple for a wide range (ceramic resonator controlled) transmitter, though it may be brought to work OK with a crystal, with some VXO functionality. A few switched crystals may provide some coverage, especially in the 40m band. I have crystals resonating on 7000, 7015, 7023, 7030 and 7040kHz, possibly 7035kHz. From box73,de 7005, 7010, 7020, 7035, 7050, 7052, 7055 and 7070 are available This should provide some coverage of the 40m CW band, so I might try that and see how much of the band can be covered with such a setup.
I see that several Xtal frequencies for the 10MHz band are available at box73.de , so a 10MHz version of the switched Xtal TX may be feasible. Frequencies available from there:
10105, 10106, 10115, 10116, 10120, 10135 and 10145kHz. This should provide good coverage of the 10MHz band with a VXO circuit.
Also from Box73.de :
80m : 3530, 3540, 3550, 3555, 3560 and 3570kHz. Generally available is also 3579kHz Partial coverage of 80m looks possible.
160m: 1800 and 1820 available from box73.de. Generally available: 1843kHz. I do have a crystal for 1963kHz.
Experiments with crystal control of the Pixie should therefore be available to me on the following bands:
- 160m
- 80m
- 40m
- 30m
While I do have crystals for higher bands (14,18,21,24 and 28MHz) I consider that the Pixie is probably not suitable for those bands. I could do a test on 28MHz, just to see what the kit does, but I do not expect good results. It may possibly work with better transistors, and with lower power.
I am still looking for crystals in the 5250 - 5450 kHz band.
Receiver experiments, with the offset diode circuit removed, should prove interesting enough. For a receiver a stabilised voltage supply can easily be arranged
Other experiments have been suggested by VK3YE on his page :
The Pixie Hack Challenge
I may try out a few of those at a later stage.
As you can see, there are still many experiments/hacks possible with the Pixie kits. That is why I purchased quite a few before starting the experimentation.
I may have a pause in the Pixie play now. Many other projects I want to do. We shall see.
Showing posts with label Ceramic resonators. Show all posts
Showing posts with label Ceramic resonators. Show all posts
2020-04-16
Pixie tests with VXO and Ceramic Resonators.
Quick test today using ceramic resonators.
40m model:
Even with a 5 - 60pF variable capacitor the frequency could be pulled down to under 7.000MHz, with a ceramic resonator for 7160kHz. The stability, however, was not sufficient with the Pixie. The tuning was fiddly, and the TX signal had a significant chirp, and the receive frequency was drifting too much for my taste. Well, it was clearly audible ...
Further, the RX offset of the oscillator, when using a crystal in series with the variable capacitor, was insufficient at the lowest frequency, even when it was set to maximum, i.e. the RX local oscillator (BFO) was too close to zero beat.
The 80m version did not fare much better. Yes, it was a bit better, but still had chirp on the TX and some drifting on the RX the RX could not be pulled down to the band edge, either, but did have a sufficient range to have been useful.
More experiments with an inductor added to the VXO (crystal) should be an interesting experiment. It might provide for a better stability, and somewhat better offset. This needs to be tested.
It may be possible to use one Pixie as RX and another as TX, providing a "spot" function, but I doubt the stability will be sufficient with the ceramic resonators.
Because VK3YE has tested oscillators and even regenerative receivers with ceramic resonators, and achieved sufficient stability for CW operation, I would consider the Pixie as too simple for good results as a frequency agile CW transceiver. It would be quite suitable for simple fixed frequency operation, e.g. as a monitor receiver for FT8. Crystals for such a receiver (20m and 40m) should arrive in about a week, if not delayed by the corona virus situation. A test of the Pixie as a simple QRSS RX should be another simple experiment.
40m model:
Even with a 5 - 60pF variable capacitor the frequency could be pulled down to under 7.000MHz, with a ceramic resonator for 7160kHz. The stability, however, was not sufficient with the Pixie. The tuning was fiddly, and the TX signal had a significant chirp, and the receive frequency was drifting too much for my taste. Well, it was clearly audible ...
Further, the RX offset of the oscillator, when using a crystal in series with the variable capacitor, was insufficient at the lowest frequency, even when it was set to maximum, i.e. the RX local oscillator (BFO) was too close to zero beat.
The 80m version did not fare much better. Yes, it was a bit better, but still had chirp on the TX and some drifting on the RX the RX could not be pulled down to the band edge, either, but did have a sufficient range to have been useful.
More experiments with an inductor added to the VXO (crystal) should be an interesting experiment. It might provide for a better stability, and somewhat better offset. This needs to be tested.
It may be possible to use one Pixie as RX and another as TX, providing a "spot" function, but I doubt the stability will be sufficient with the ceramic resonators.
Because VK3YE has tested oscillators and even regenerative receivers with ceramic resonators, and achieved sufficient stability for CW operation, I would consider the Pixie as too simple for good results as a frequency agile CW transceiver. It would be quite suitable for simple fixed frequency operation, e.g. as a monitor receiver for FT8. Crystals for such a receiver (20m and 40m) should arrive in about a week, if not delayed by the corona virus situation. A test of the Pixie as a simple QRSS RX should be another simple experiment.
2020-04-12
Pixie Transceiver Kit Building and Mods.
Over the pas year or two I have eBay'ed quite a few kits of the simple Pixie CW transceiver. Now was the time to build and test a few, one as the original and one modified.
All those kits are so cheap from Chinese eBay'ers that I can buy 3, sometimes 4 kits for the tax free limit to Denmark (about 12 - 13 USD). I purchased a few, then another few. For that low price the kits are excellent for experimenting.
More modified will be built, for all the lower bands, but here is the first impressions:
Kit #1: 40m CW on 7023kHz:
The kit is easy enough to assemble. I made good use of my PCB holder for stability.
Resistor marking was good old standard. Some markings were missing on capacitors, so they were checked. The inductors were checked, as well, as I was not sure of the colour coding for those.
First impressions of the finished kit. It was expected to be tricky making contacts with such a low power device only capable of operating on a single TX frequency.
Power output, measured with a SWR/power meter is estimated to be just over 300mW. A trim-potmeter varies the oscillator frequency for receiving, so a decent "side tone" can be achieved. For receiving an old speaker was connected. A low hiss was audible, but the audio level was too low for comfortable operation. Further, some odd noises and distortion were heard (more about that later).
The first test was done with a local friend and my low hanging HF dipole, at the massive distance of 3km (about 2 miles) ;) Reports received were 599 (yes, for real), but I could only give 539, most likely due to the low audio level.
The strange noises were identified, the LM386 did oscillate, apparently triggered by strong signals received. Not good.
An Internet search revealed that this is a general problem with the LM386 in this configuration when connected to a low impedance (8ohm) speaker. A test with connecting the to a set of active computer speakers showed no oscillations or distortion at all. So much for the simplicity of the kit.
With the extra gain of the computer speakers the receiver is more lively, too.
Kit #2: 80m CW on 3560kHz:
Components from the original (2nd) kit, and some components from a third kit were used like this:
the values of the capacitors and inductors in the RF part of the circuit were doubled in value. The inductors from the 2 kits were connected in series, and the capacitors in the oscillator resonant circuit and the low pass filter were coupled in parallel. Interesting enough, it was quite easy to fit the parallel capacitors in the mounting holes.
That was all. Connected to the computer speakers the receiver sprang to life, and the sensitivity seemed OK, the noise level increased a bit when the antenna was connected.
My friend was not available at the first test, so no test QSO was made (yet)
Power output of the modified 80m was close to 500mW at 9V power supply. A bit more than the 40m version, but not surprising.
Running a CW transceiver at low power, and at single frequencies is, of course an exercise in frustration, so the next test will be modification of both kits for some frequency agility.
For the 80m version the first test will be replacing the crystal with a ceramic resonator and a variable capacitor. This should provide coverage of a good part of the 80m CW band.
The 40m version is a bit more tricky The resonator available is a 7159 version, and might not be capable of covering the CW band portion of 40m. I do have a 7.02MHz ceramic filter, and that might cover a part of the 40m CW band. Otherwise a VXO with some switched crystals available will be sufficient. Crystals available to me are 7.000, 7.015, 7.030 and 7.040MHz. With those I would expect to cover most of the 40m CW band.
It should be possible to make versions for the 6m, 160m and 630m bands, with other modifications. I should have the necessary replacement components available, but that is for later.
Update: The 80m version was tested today, and provided a 599+10 report from my local friend. It works, most definitely.
All those kits are so cheap from Chinese eBay'ers that I can buy 3, sometimes 4 kits for the tax free limit to Denmark (about 12 - 13 USD). I purchased a few, then another few. For that low price the kits are excellent for experimenting.
More modified will be built, for all the lower bands, but here is the first impressions:
Kit #1: 40m CW on 7023kHz:
The kit is easy enough to assemble. I made good use of my PCB holder for stability.
Resistor marking was good old standard. Some markings were missing on capacitors, so they were checked. The inductors were checked, as well, as I was not sure of the colour coding for those.
First impressions of the finished kit. It was expected to be tricky making contacts with such a low power device only capable of operating on a single TX frequency.
Power output, measured with a SWR/power meter is estimated to be just over 300mW. A trim-potmeter varies the oscillator frequency for receiving, so a decent "side tone" can be achieved. For receiving an old speaker was connected. A low hiss was audible, but the audio level was too low for comfortable operation. Further, some odd noises and distortion were heard (more about that later).
The first test was done with a local friend and my low hanging HF dipole, at the massive distance of 3km (about 2 miles) ;) Reports received were 599 (yes, for real), but I could only give 539, most likely due to the low audio level.
The strange noises were identified, the LM386 did oscillate, apparently triggered by strong signals received. Not good.
An Internet search revealed that this is a general problem with the LM386 in this configuration when connected to a low impedance (8ohm) speaker. A test with connecting the to a set of active computer speakers showed no oscillations or distortion at all. So much for the simplicity of the kit.
With the extra gain of the computer speakers the receiver is more lively, too.
Kit #2: 80m CW on 3560kHz:
Components from the original (2nd) kit, and some components from a third kit were used like this:
the values of the capacitors and inductors in the RF part of the circuit were doubled in value. The inductors from the 2 kits were connected in series, and the capacitors in the oscillator resonant circuit and the low pass filter were coupled in parallel. Interesting enough, it was quite easy to fit the parallel capacitors in the mounting holes.
That was all. Connected to the computer speakers the receiver sprang to life, and the sensitivity seemed OK, the noise level increased a bit when the antenna was connected.
My friend was not available at the first test, so no test QSO was made (yet)
Power output of the modified 80m was close to 500mW at 9V power supply. A bit more than the 40m version, but not surprising.
Running a CW transceiver at low power, and at single frequencies is, of course an exercise in frustration, so the next test will be modification of both kits for some frequency agility.
For the 80m version the first test will be replacing the crystal with a ceramic resonator and a variable capacitor. This should provide coverage of a good part of the 80m CW band.
The 40m version is a bit more tricky The resonator available is a 7159 version, and might not be capable of covering the CW band portion of 40m. I do have a 7.02MHz ceramic filter, and that might cover a part of the 40m CW band. Otherwise a VXO with some switched crystals available will be sufficient. Crystals available to me are 7.000, 7.015, 7.030 and 7.040MHz. With those I would expect to cover most of the 40m CW band.
It should be possible to make versions for the 6m, 160m and 630m bands, with other modifications. I should have the necessary replacement components available, but that is for later.
Update: The 80m version was tested today, and provided a 599+10 report from my local friend. It works, most definitely.
2020-02-25
Second Crystal/Ceramic Resonator Tester. Update
I built a second crystal tester today. This time I used 470pF capacitors for the Colpitts oscillator "resonating capacitance", and a larger transfer capacitor.
As expected, this worked nicely with ceramic resonators (but not 3-pin filters) down to the lowest available, 400kHz, and up to about 2MHz (crystals). One 1843kHz crystal did not work, but a 1963kHz crystal did. Maybe the 1843 crystal was poor quality.
I expect that I should build the third one of of the crystal testers, this time with 100pF "resonating capacitance". This **should** provide tests the intermediate frequency range of 1.8 - 8MHz, at least for crystals and ceramic resonators.
All in all I am happy with the results, I was warned that the crystal tester kit, advertised as 1 - 50MHz crystal tester, was not working in the full range, so I had a few kits taken home.
I could see that one of the ceramic resonators (marked 480kHz) oscillated on 476kHz, i.e. inside the 630m band. I suspect that I can build a VXO with one of those resonators, covering the full 472 - 479kHz band. Oops ! Yet another possible project to try out. Maybe a modified Pixie kit can be brought to work on 472kHz - I would not be surprised.
Update:
A third version with 2x 100pF in the Colpitts oscillator was built. This appears to work nicely from 2 - 16MHz with crystals.
Ceramic resonators:
- The 3.5 - 4MHz range works well with some resonators, no filters.
- The 7.16MHz (3-pin) oscillates nicely just under 7MHz with the capacitors in the oscillator.
- 12MHz resonators oscillate fine.
- 5.5MHz filters do not oscillate at all.
My conclusion is that the three testers combined will provide some crystal and ceramic resonator/filter tests, but a dedicated (set of) oscillator(s) and a dedicated frequency counter with high impedance/low capacitance input is most likely necessary for a better test system.
As expected, this worked nicely with ceramic resonators (but not 3-pin filters) down to the lowest available, 400kHz, and up to about 2MHz (crystals). One 1843kHz crystal did not work, but a 1963kHz crystal did. Maybe the 1843 crystal was poor quality.
I expect that I should build the third one of of the crystal testers, this time with 100pF "resonating capacitance". This **should** provide tests the intermediate frequency range of 1.8 - 8MHz, at least for crystals and ceramic resonators.
All in all I am happy with the results, I was warned that the crystal tester kit, advertised as 1 - 50MHz crystal tester, was not working in the full range, so I had a few kits taken home.
I could see that one of the ceramic resonators (marked 480kHz) oscillated on 476kHz, i.e. inside the 630m band. I suspect that I can build a VXO with one of those resonators, covering the full 472 - 479kHz band. Oops ! Yet another possible project to try out. Maybe a modified Pixie kit can be brought to work on 472kHz - I would not be surprised.
Update:
A third version with 2x 100pF in the Colpitts oscillator was built. This appears to work nicely from 2 - 16MHz with crystals.
Ceramic resonators:
- The 3.5 - 4MHz range works well with some resonators, no filters.
- The 7.16MHz (3-pin) oscillates nicely just under 7MHz with the capacitors in the oscillator.
- 12MHz resonators oscillate fine.
- 5.5MHz filters do not oscillate at all.
My conclusion is that the three testers combined will provide some crystal and ceramic resonator/filter tests, but a dedicated (set of) oscillator(s) and a dedicated frequency counter with high impedance/low capacitance input is most likely necessary for a better test system.
2020-02-24
Test Equipment: Crystal Tester Kit.
I just assembled a low cost Chinese kit claiming to test crystals from 1 - 50MHz, with a counter w/5-digit display). Assembly time, taking things easy, was about 1 hour.
I already suspected that the kit would not work down to 1MHz, for this reason: The crystal oscillator is a Colpitts oscillator with the capacitances across the crystal (B-E and E-GND) being only 22pF each. The capacitances in the Pixie kit's oscillators is much higher, and that was designed for 7MHz.
This was confirmed when testing crystals. Below 5MHz hardly any crystals would generate a countable signal. However, crystals up to 28MHz would generate countable signals.
I suspect that a different tester with, say 100pF capacitors would likely work down to about 2MHz, and 470pF capacitors down to about 400kHz.
I do have another kit or two, so I will probably build those with the above mentioned modifications.
One more test was done: Trying out ceramic resonators. Resonators (2-pin) or filters (3-pin) were tested. None worked on less than 10MHz in this circuit, but did work up to 20MHz, with rather consistent results for the resonators (2-pin), and inconsistently with some of the filters (3-pin). I suspect the low capacitance values are the reason for this.
Also, a more suitable oscillator type for the filters should be tested.
I may end up with a counter with an 8-digit display and some dedicated oscillators as my crystal tester. This could be interesting for testing frequencies for crystals to be used in crystal filters. It is likely that a set of modified Pixie oscillators would do the job.
Finishing one project creating more possible projects. Why am I not surprised.
I do have a few Pixie kits lying around, so maybe the next project should be an unmodified Pixie transceiver, with the exception of using an external crystal or ceramic resonator for the oscillator.
I already suspected that the kit would not work down to 1MHz, for this reason: The crystal oscillator is a Colpitts oscillator with the capacitances across the crystal (B-E and E-GND) being only 22pF each. The capacitances in the Pixie kit's oscillators is much higher, and that was designed for 7MHz.
This was confirmed when testing crystals. Below 5MHz hardly any crystals would generate a countable signal. However, crystals up to 28MHz would generate countable signals.
I suspect that a different tester with, say 100pF capacitors would likely work down to about 2MHz, and 470pF capacitors down to about 400kHz.
I do have another kit or two, so I will probably build those with the above mentioned modifications.
One more test was done: Trying out ceramic resonators. Resonators (2-pin) or filters (3-pin) were tested. None worked on less than 10MHz in this circuit, but did work up to 20MHz, with rather consistent results for the resonators (2-pin), and inconsistently with some of the filters (3-pin). I suspect the low capacitance values are the reason for this.
Also, a more suitable oscillator type for the filters should be tested.
I may end up with a counter with an 8-digit display and some dedicated oscillators as my crystal tester. This could be interesting for testing frequencies for crystals to be used in crystal filters. It is likely that a set of modified Pixie oscillators would do the job.
Finishing one project creating more possible projects. Why am I not surprised.
I do have a few Pixie kits lying around, so maybe the next project should be an unmodified Pixie transceiver, with the exception of using an external crystal or ceramic resonator for the oscillator.
2019-12-21
Idea Box: QSOs With Very Simple Home Made Equipment ?
Making QSOs with modern manufactured equipment is fun, but what about doing it with home made equipment? (maybe not making your on components, such as capacitors, but using existing available components, and maybe sometimes modules. More fun ? I think so.
I have made my own direct conversion receiver for 80m, and later a simple VXO-transmitter for 40m. Making a few QSOs with that TX was absolutely fun. But what about making QSOs with fully home mad transmitter/receivers. Even more fun.
I have got the idea of trying to make at least one QSO on as many bands as possible, with homemade or at least modified surplus modules or equipment, as simple as possible.
On a few bands there are excellent simple options:
With ceramic resonators (CR) it should be possible to build relatively simple transmitters with the resonators "pulled" like it is done with crystals in VXOs, and then keying the buffers/amplifiers.
Many people have done this. As someone once said "a transmitter is 'just' an amplifier". Yes, but one of the stages is unstable, and oscillates ;)
But what about receivers ?
Direct conversion reception is often done with simple CW transmitter/receivers, but could it be even more simple ?
Today I was watching a video by VK3YE. He demonstrated a QSO made with a CR controlled transmitter on 40m (80m easily done, too, with CRs available). and here is the trick: He was using a simple regenerative receiver, set just above the point of oscillation. That makes it possible to listen to CW/SSB signals with a 3-transistor receiver (using an earphone). This is quite well known as well, but the regenerative receiver can be very critical and is considered a "two-hand" receiver. One hand on the tuning, the other on the regeneration control potmeter.
I consider this type of receiver as a direct conversion receiver with a self-oscillating mixer, although it is not classically considered as such.
VK3YE's trick was using a CR for controlling the frequency of the regen-receiver. The result is a far less critical regeneration control, and a far better frequency stability. How about a receiver with no coils to wind? Here it is.
On one of his videos he demonstrated a DX QSO with a CR controlled 30W TX and the above mentioned RX.
Doing this does mean that it is necessary to go "back in time" and use separate frequency tuning for the TX and RX, and having a "spot" function, so the TX frequency (just the oscillator) can be heard in the receiver.
No, I would **not** build such a set with vacuum tubes, though I know that it is quite possible. I do not like high voltages.
So many ideas, so little time, but it is a tempting project to try out after a few other things I would like to finish. Beginning projects is a lot easier than finishing ;)
I have made my own direct conversion receiver for 80m, and later a simple VXO-transmitter for 40m. Making a few QSOs with that TX was absolutely fun. But what about making QSOs with fully home mad transmitter/receivers. Even more fun.
I have got the idea of trying to make at least one QSO on as many bands as possible, with homemade or at least modified surplus modules or equipment, as simple as possible.
On a few bands there are excellent simple options:
With ceramic resonators (CR) it should be possible to build relatively simple transmitters with the resonators "pulled" like it is done with crystals in VXOs, and then keying the buffers/amplifiers.
Many people have done this. As someone once said "a transmitter is 'just' an amplifier". Yes, but one of the stages is unstable, and oscillates ;)
But what about receivers ?
Direct conversion reception is often done with simple CW transmitter/receivers, but could it be even more simple ?
Today I was watching a video by VK3YE. He demonstrated a QSO made with a CR controlled transmitter on 40m (80m easily done, too, with CRs available). and here is the trick: He was using a simple regenerative receiver, set just above the point of oscillation. That makes it possible to listen to CW/SSB signals with a 3-transistor receiver (using an earphone). This is quite well known as well, but the regenerative receiver can be very critical and is considered a "two-hand" receiver. One hand on the tuning, the other on the regeneration control potmeter.
I consider this type of receiver as a direct conversion receiver with a self-oscillating mixer, although it is not classically considered as such.
VK3YE's trick was using a CR for controlling the frequency of the regen-receiver. The result is a far less critical regeneration control, and a far better frequency stability. How about a receiver with no coils to wind? Here it is.
On one of his videos he demonstrated a DX QSO with a CR controlled 30W TX and the above mentioned RX.
Doing this does mean that it is necessary to go "back in time" and use separate frequency tuning for the TX and RX, and having a "spot" function, so the TX frequency (just the oscillator) can be heard in the receiver.
No, I would **not** build such a set with vacuum tubes, though I know that it is quite possible. I do not like high voltages.
So many ideas, so little time, but it is a tempting project to try out after a few other things I would like to finish. Beginning projects is a lot easier than finishing ;)
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