I finally got around to building up the new version after I had ordered the wrong chips (MSOP instead of SOIC).
Results so far:
- I goofed up the input of the first prescaler (again)
: C22 has to be replaced with a solder bridge and pins 2 and 3 of U4 have to be connected with a 1 kOhm resistor. I have soldered a 0603 resistor across the legs, but soldering a resistor to C21 and the solder bridge that replaces C22 works too.
- Frequencies up to 2.89 GHz can be measured at input-levels of -30 dBm.
- Frequencies lower than 290 MHz cannot be measured reliably and anything lower than 350 MHz requires at least -10 dBm to be measured reliably.
My investigation so far points to 3 possible causes:
- Signal Source. I'm using the ADALM Pluto in CW-Mode and the spectrum looks horrible with powerful wideband spurs close to the CW-Signal, as well as strong harmonics. The output has massive jitter which gets really bad at
frequencies below 280 MHz. At 100 MHz the jitter takes up almost 25% or a full period.
- Oscillations in the second amplifier: I noticed 1.2 GHz parasitic oscillations that become quite strong and frequency stable when a finger is placed over U2, the output-trace of U2 and the Bypass-Caps C4 and C5. This is rather
surprising,
since until now I never had any oscillation-problems with the BGA2869 Amplifier and used it in several projects without any problem. Even mounted dead bug-style on a vero-board it didn't oscillate.
- A problem with the splitter (R2, R4 and R5 arranged in a Delta-Configuration) and/or reflections from the RF-Detector for the standby-Circuit.
I've also noticed a tendency to count harmonics if the input-signal has a frequency of less than 100 MHz. Interestingly, I've had absolutely no issues when I fed the prescaler a 24 MHz-Signal from my FeelTech DDS-Signal-Gen...
So 2 steps forward (higher sensitivity, frequency-range extended), one step back (Problems with the Divider-Chips again).
Sorry @ geostep and everyone else who bought one of the new boards. I really thought I had nailed it this time
Any ideas on how to fix or how to tackle this problem?
My big problem is that I don't have a good working signal source for frequencies between 24 and 900 MHz.