Alison KB1GMX has been exploring ways to flatten the TX output across bands over the last couple of weeks, but says,
“Here is the problem… Improving the amp leads to acknowledging other issues. In particular … the spurs!”
When transmitting on 15m and 10m the LPF has a cutoff frequency over 30 MHz, so effectively the amp will pass everything through below 30 MHz.
If you overdrive the mixers especially the 45mhz output mixer you get more than one product from the mixer. For a desired output at 28 MHz, you actually get two other outputs, one at 11.995 MHz (that is 50 to 60db down on the desired output) and another at about 17 MHz, which if the mixer is not over-driven is around 40dB down. However, if the audio is pushed a bit harder, it may be only 25db down from the desired frequency. The amplifier has no filtering below 30MHz and will gladly amplify all of the signals through the pre-driver, driver and finals stages and pass them out to your antenna.
This is not an issue until you hit around 18 MHz as the output low pass filters that are switched in help attenuate the spurs.
The Maths!
Output is IF(about 45mhz) + LO (48 to 75mhz)= 3 to 30 mhz
The spur case is IF(45mhz) – Output frequency (42 to 15mhz) Its going down while the dial frequency is going up.
For up to 18mhz the output lo pass filters and the inside 33mhz low pass filter are at work doing the job.
However for 21 and 28 MHz both filters are roughly 33-35 MHz cutoff and the spur is now down around 24 and 17 MHz respectively. They will be about 40-50db down until the point where the 45mhz mixer overloads and then the spurs get very much stronger at a frequency well below the cutoff of the low pass filters. Since power output is low on 10M people will likely push the audio and the spur will get significantly worse.
The is due to the nature of Double Balanced Mixers: they are three port devices and all three can receive input or deliver outputs. Also inside the mixer all possible sums and differences for the base frequencies and their harmonics will exist.
This was determined, without the amp operating, by breaking the circuit at C200 to allow a spectrum analyzer to be inserted. We can’t blame the amplifier chain for this as it would just do its designed duty and amplify everything.
There is no obvious solution at present
Right now Allison has no obvious solution to present.
Commercial radios implement a switched filter at the point were L1 though L4 (and C200 to C204) are present and a filter would be switched in for 15M, 12M and 10M. Allison is investigating other ideas.
This issue does not show up in CW mode as the radio is keyed in a different manner and there is no 45 MHz contribution.
Additional comment
Jerry KE7ER comments that the spur is not a problem from 3.5 MHz up to 18MHz. So for operators that never use the 15, 12 and 10 meter bands on SSB, this should not of concern. For many of us, a $129 rig that covers 80-17 meters is still a very good deal.
For those wishing to use the rig above 18 MHz, perhaps some sort of external band-pass or high-pass filter is in order between the µBitx and the antenna.
Some (but not all) antenna tuners might be sufficient.
If the audio from the mike amp is kept reasonable, Allison reports that the spur is 40+ dB down. Jerry assumes it should be 43 dB below the primary signal to be compliant in the US, so it’s marginal at best. The worst of this problem could be avoided by somehow monitoring the transmit signal level at the mixers. Allison has found that gain with the 2n3904’s can vary on the upper bands depending on your particular device characteristics, so it currently is not sufficient to just check the level at one point in the chain. However, if gains can be made more predictable, perhaps monitoring at the top of RV1 using a diode RF probe would be sufficient?
The µBitx has a 30 MHz low pass filter between the mixers and the power amp. As Allison says, most multiband rigs would have switched bandpass filters in that position. Signals are quite low level, so this could probably be done with analog switches rather than relays. So yet another possibility is to add a daughterboard with switched bandpass filters to replace the filter at L1,2,3,4. We only need 3 or 4 filters there and parts can be small and cheap, so not so bad.
The spur is at the first IF frequency of 45 MHz minus the operating frequency. If the first IF is raised from 45 MHz to 70 MHz or more, that should remove the spur for all frequencies from 3.5 to 30 MHz. That 70MHz IF strip would definitely require something better than the 2n3904’s that are in the µBITx!
