DC VXO transceiver performance criteria?  - Page 1
Forum section: electronics / rf-microwave

author: rhb
author_rank: Super Contributor
timestamp: July 23, 2020, 12:12:49 am
content: I'm interested in designing ham transceivers. To start with I've built a Chinese 40 m "Pixie" VXO transceiver kit which is 2 transistors and an LM386 AF amplifier. The design dates from when the FCC exempted QRP gear from the harmonic suppression regulations. So as being sold the kit does not meet current regulations. Fixing it is easy, but it seems not to have been done by most or all sellers.I'm making up a list of things to measure. There are many things one can measure, but many don't apply to a simple design such as the Pixie. So far I have:minimum detectable signalRF output vs input voltageminimum operating voltagecurrent drain Rx & Tx vs input voltageVXO tuning rangeoutput harmonic levelskeying waveformaudio output vs RF signal and input voltageselectivity and adjacent signal dynamic rangeThe first measurements I'm confident I know how to do, but the last two are a bit more uncertain, especially the last.My primary test gear for this effort is:Keysight 33622A 2 channel 120 MHz AWG (for all intents unmeasurable harmonic output)HP 8560A spectrum analyzer w/ TGHP 438A power meter w/ 8482A sensorHP 5386A 3 GHz frequency counter2x HP 3478A DMMs for voltage and currentInstek 200 MHz DSO and/or Tek 485HP 8970B noise figure meter and 346B noise sourceHowever, I have lots of other gear available and no qualms about buying something I don't have if needed.Does anyone have other measurements to suggest which are relevant to the performance of an ultra portable transceiver design? Maximum safe SWR seems a likely candidate, but I have no idea how to actually measure that without destroying the PA transistor. I'll be going through SSD and EMRFD looking for ideas, but if someone can suggest specific sections to read I'd appreciate the references as well as any others.I'll be doing the initial measurements manually, but plan ultimately to automate them under GPIB control to simplify making plots such as current drain vs input voltage and to ensure that all the tests are consistently performed. I plan to develop similar protocols for other radio designs such as superhets and the single transistor set discussed earlier.Have Fun!Reg

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: August 10, 2020, 11:16:22 pm
content: All that's too technical.My criteria in picking a design is whether it fills the 'sweet spot' where it is good enough to make contacts but not too complex.A lot of numbers are irrelevant in the 'real world'. Eg If there is a significant increase in band noise when you connect the antenna then you know the receiver is sensitive enough.This video describes what you need in QRP direct conversion transceivers. Yes, the Pixie falls short.More detail is on my website here:http://home.alphalink.com.au/~parkerp/qrp/equipment.htmTo summarise you need at least 2w, a receiver that doesn't fall to bits when there are strong signals (depends on your area - in most even an NE602 design will be OK but in others you might go for a diode balanced mixer) and a good VXO range as you'll be searching for stations to work (most of the time). FREQUENCY AGILITY IS KING! Then there are highly desirable things like a predictable t/r frequency offset (which I address in the video).

author: rhb
author_rank: Super Contributor
timestamp: August 10, 2020, 11:56:12 pm
content: On analysis the Pixie is proving pretty sad.~10 mW key up LO radiationNo VXO except RITNo selectivityHowever, after a few days of analysis of various aspects, I think these issues can be resolved by good engineering.I did get all harmonics <-60 dB below fundamental by changing from a Butterworth to a Cauer filter by using a 4 part LPF instead of a 3 part LPF.I'm a hardcore Lord Kelvin guy. If you can't measure it, whatever knowledge you have is rather incomplete. After watching your video, I'd simply remark "necessary, but not sufficient".Have Fun!Reg