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.
Showing posts with label Crystals. Show all posts
Showing posts with label Crystals. Show all posts
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.
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