Has anyone seen measurements of this TCVR on a spectrum analyzer? I was wondering if it can handle a PA connection as well. Somehow, despite the expansion of this TCVR (including RS-918, RS-928,..), I haven't found this. TNX
Posted In: Technique
mcHF IMD and signal purity
- ParticipantParticipantReply
The problem is that there are several revisions of the MCHF, several adjustments of the final stage, triggering, several types of chips in the 'switch', several types of RX/TX switching, and other changes. It is also necessary to correctly set the IQ for TX, power, idle currents, and pair transistors... And finally, each band has different results, it has 'only' 4 BPF/LPF, so the results will be different on warcos and different on 'classic'. I have a radio, modify tens (original version 0.4) and when comparing antennas 35m apart it is at the level of FT-817, PA up to 50W from portable would handle it, but something stronger probably won't be kosher. Measurement of filters and adjustments I have here a z lepsich anten uz koser asi nebude. Meranie filtrov a upravy mam tuhttps://sites.google.com/site/om1aeg/mchf
ParticipantReplyTNX, Tóno! I have already looked at the measurements on your page and also the experiences of OM6IN http://om6in.blogspot.com/ (also Tulip). Probably yes, but it would be interesting to take a look at it.
I have uBITX (also with modifications) and there were practically 3 significant adjustments in the TX path (modification/change of amplifiers, filtering of unwanted products behind them, and finally replacing LPF relays with types with greater isolation between contacts).
ParticipantReplyVilo, snr 2019.
You know Lubo's pages, on one link there are measurements of the output signal (even if only the main signal, not harmonics), http://om6in.blogspot.com/2017/02/uprava-obv from u-vysielania.html?m=1 As you can see, what version, adjustments, and settings, that's another signal 😉
I plan to make adjustments around 'bias' for switches, add chokes to the microphone input (at 80m under the antenna on sota it caused interference at 15W) and also measure what comes out of it. However, I only have the option to measure above 21MHz with an airspy mini key, I need to add an output with attenuation to the kv artificial load. I will finish the 'project' Zello and Echolink for OM0OUB and RX Zello OM0OVV and I will focus on that.
ParticipantReplySo I added a -50dB output to the KV artificial load. Connected airspy mini, test with SDRSharp and SpectrumSpy SW. Unfortunately, SpectrumSpy works only from 25MHz.
Test at 29.5MHz, FM, set to 5W, without modulation, but with the microphone connected. Power supply LiPo.
MCHF according to the wattmeter during the test 4.3W
FT-817 according to the wattmeter during the test 4.1W
Results: https://photos.app.goo.gl/7Znrf8ioxgHmYJV9AThe third harmonic is a problem on FT-817
On MCHF, the spectral purity is around, I will play and try to improve it.My MCHF is definitely not suitable for PA at the moment 🙁
ParticipantReplyWow, that's definitely not a very nice signal 🙁 Could it just be some setting?
ParticipantReplyI will look for where the betrayal is, I already have a high-impedance VF probe with a 50R output for SDR / VNA, so I will check where the problem is. Until now, I didn't measure the output, so I lived contentedly 🙂 I only use MCHF from a portable, in 2018 it was used 5 times for tens of minutes, so maybe I didn't make such a pass from a compromise stretched dipole. Anyway, I was going to adjust the "bias" for the switches and the RX preamp, so there will be more reason to disassemble it.
ParticipantReplyI was interested, because I think the receiver is excellent + the possibilities that SDR provides (filters, spectroscope) are certainly welcome. If it were clean, I could imagine supplementing the mcHF with PA, which would create a very competitive TCVR with conventional devices.
I don't know if it's the specifics of the setting or the features of the construction...
ParticipantReplyOn April 4, 2020, I rewired the TX path of the mcHF TRX and changing the modulation method from a 4-switch to a dual-switch significantly helped. Before the modification, the suppression of the first 'parasite' was only 20dB against the 'carrier' and another parasite 42dB @28MHz. After modification, the suppression of the strongest 'parasite' is 44dB against the 'carrier'. The schematic and measurements at https://photos.app.goo.gl/5hZW7GWuWJUHqFev5
AdministratorReplyWouldn't it be more appropriate to measure it with a two-tone test? https://om0a.cq.sk/amaterske-radio/ubitx-imd-modifications/
Because if I look, I see a strong carrier in the middle, that won't be 'TX Carrier suppression' and the others may also be generated by an oscillator at some clock, which may be related to FM.
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