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You could probe the input to the 3.3v regulator. However, this is a little more complicated than what it appears to be. First, I spent weeks measuring the spurs. They are all below -43dbc except on 21 mhz. To quote a politician: "those are my alternate facts". But, we must first be a little careful in trusting the tinysa. About 10 years ago, I built a spectrum analyser. The amount of shielding it needed was insanely high. See?. The spectrum analyzer needs to be carefully shielded at all stages to be useful, esp with a transmitter testng. The signal from the antenna terminal is reduced at least 60 dB before being applied to the spectrum analyzer. On the other hand, there are many other ways that the RFI can leak from the radio into the spectrum analyzer at any stage and produce these heterodyne beats. The TinySA is a single PCB with mixed signals without any shielding.? The zbitx is a superhet radio with a number of oscillators apart from just the Si5351. There is a 12 MHz crystal on the WM8731, a TCXO at 25 MHz, many spurs coming out of the RPI, the display and the Pico. However, the diode mixer of the zbitx was chosen as a doubly balanced mixer that has far fewer spurs than the Kiss Mixer of the sbitx. This is borne by direct measurements?as well. The uBitx too uses exactly the same front end (of diode mixers driven by an IF amplifier) in transmit, it has acceptable spurious emissions. For a proper testing, you need a very well shielded or a professional quality instrument. Both my spectrum analyzers, the specan and the Rigol DSA-815 are those. My tests were done on both.? I have also noted a correlation?between the bias current and the spurs. Here are my settings on the on the test bench with the DSA-815: The output of the TX is connected to a dummy load made from 20 x 2 W, 1K carbon resistors. A sniffer consisting of a 100K in series with a 50 ohms is used to tap the input into the DSA-815.? The Spectrum Analyzer is set to 30 KHz resolution bandwidth, span of 70 MH, the zbitx mode is set to CW, straight key. These are the recommendations of the ARRL's testing lab.?? I will repeat these tests and post the pictures tomorrow. On Wed, 16 Apr, 2025, 6:57?pm Lee via , <kx4tt=[email protected]> wrote:
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