Hello everyone,
I have TSMU unit, which has the same RF-block as FSH3.
It is broken, i.e. it can't measure correctly. I will be very gratefull, if someone help me to repair it.
My TSMU-RF-board PCB is older than FSH3, just a little difference at the input (other SPDT RF-switches=AS179-92LF, SMD marking S79) and other/older TCXO. Attenuator IC is also missing.
2xRF-switches + 1xBFP520 are making the circuitry of Pre-Amplifier, placed between missing Attenuator-IC and 3GHz-LPF. Pre-Amplifier can be switched ON or OFF/by-passed by the software.
RF-transistors used are: BFP420 (SMD marking AMs) and BFP520 (SMD marking APs).
Below is the progress until now on my attempt to repair the unit.
Due to the info, found in FSH3/6 service manual and also in this thread, I've made some investigations and measurements. I think there is a problem with PLL-synthesizer: it can't lock correctly.
I made that decision by measuring the signal at the output of IC=HMC434 (divide-by-8 module, placed between 1st-LO and SYNCON-synthesizer, i.e.
between 2xBFP520-transistors and IC=uPB1509GV/divide-by-2 or 4 or 8 module) and the signal at the output of IC=HMC433 (divide-by-4 module, placed between 2nd-LO-3200MHz (1xBFP420-transistor) and 3rd-LO-800MHz (1xBFP520-transistor).
The signal at IC=HMC433 output is OK: exact 800 MHz, with strong level. But the signal at IC=HMC434 output is not OK.
Due to RF block diagram, 1st-LO-frequency is between 4031...7031 MHz, and after HMC434 it must be between 503...879 MHz.
So:
1. if I want to measure signals with frequency=925...960 MHz (=signals from BTS->Mobile at 900MHz-band), 1st-LO-frequency after HMC434 must be between 619 and 624 MHz. In my case this is not true, I don't observe increase in levels of this spectrum piece.
2. if I want to measure signals with frequency=2110...2170 MHz (=signals from BTS->Mobile at 2100MHz-band), 1st-LO-frequency after HMC434 must be between 767 and 776 MHz. In my case this is not true, I don't observe increase in levels of this spectrum piece.
For the measurements, I'm using Tektronix NetTek Spectrum Analyzer YBT250, with frequency range 30...2500 MHz.
I also found, that 1st-LO is made by 3xVCOs: 1xBFP420, 1xBFP520, 1xBFP520, and each one is responsible for a spectrum piece of about 1000 MHZ (VCO-1=4000...5000, VCO-2=5000...6000, VCO-3=6000...7000 MHz). Each VCO is switched-on by applying Vcc to respective BFPx20-transistor, and the exact frequency depends on Vtune, applyed to the varicaps (a pair for each VCO, Vtune rail is common for all VCOs). I observed the above facts by using ROMES software to measure at 3 different frequencies: 500, 1500, 2500 MHz, which in turn pulls into operation the respective VCO. Scanning with ROMES software at full band 15...3000 MHz didn't give any increase of levels for spectrum 503...879 MHz at HMC434-output.
It seems that VCOs are working (Vtune is ramping to about 20V when full band 15...3000 MHz is selected for measuring), but to be 100% sure, I need spectrum analyzer with higher bandwidth- at least up to 7...8 GHz.
So, at this moment, I don't know if the problem is in HMC434.
The only thing, which I can try at the moment, is to swap the places of HMC434 and HMC433. In such way, I can test if HMC434 works properly: at HMC433's place (after 2nd-LO-3200MHz), the output of HMC434 must be 400MHz.
Any suggestions?