For the HMC703, what should the DC bias level be on the XREFP pin(20)? If it is too high, what is probably cause?
Check you are not exceeding the minimum N restriction. You only need to turn on the RF input divide-by 2 prescaler when the VCO frequency exceeds 4GHz. For operation below 4GHz, assuming fractional-N mode, the HMC703 RF divider value must be >= 20. If you want to lock to 1GHz the reference frequency must be <=50MHz (assuming R=1).
The reference buffer is powered from the RVDD (pin 18) 3.3V power rail. I measure 0.46V on pin 20 when the buffer is enabled (Reg 0x08=1) and zero when disabled (Reg 0x08=0). The reference input requires an external dc blocking capacitor.
Thanks Dyoung1, I appreciate the help. My voltages were off probably due to a paddle ground that was disconnected. The levels and offset look correct now.
The main problem I'm having is with the HMC703 running with a wide band VCO ~1-2 GHz. The charge pump output seems to be clamped ~0.09V regardless of whether or not the vco tuning line is disconnected. With another functioning higher freq ~6GHz unit, the charge pump output matches the op amp loop filter non-inverting input (mid rail) in closed loop even before programming.The working synth charge pump output jumps to 5V synth rail when loop is disconnected by breaking tuning line connection.
Is there a change in programming requirement that I'm not aware of for operation with a 1-2 GHz VCO?
Also, the harmonic content of the VCO is quite high, about -13 dBc for 2fo. Is that going to be an issue for the frequency/phase detector?
I have tried disconnecting the tuning line and feeding in an external tuning voltage while monitoring the charge pump output with a DMM. I don't see any movement in the charge pump output.
I was able to get the synthesizer working from 1.9 to 2 GHz, but I could only cover the rest of the VCO range if
I set the RF divider to bypass. Also, the type A modulator seems to have spurs 1.1 MHz away from my carrier vs.
the type B modulator. Does this sound correct? I'm not sure why I would have to bypass the RF divider to enable
operation in the lower ranges.Thanks, Gerald
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