Dual stage down conversion for PLL, how are LO frequencies chosen? - Page 1
Forum section: electronics / rf-microwave

author: Staffan321
author_rank: Regular Contributor
timestamp: January 04, 2020, 05:30:31 pm
content: Hello,I'm playing around with a VNA where frequency is locked to a 10 MHz reference through a two stage down conversion. First stage is an harmonic mixer based on step recovery diodes fed by an LO in the range 500 - 540 MHz (roughly) so that the second stage can mix down from 85 - 95 MHz using an LO at 82.5 - 92.5 (roughly) to get the 2.5 MHz that is fed to the phase comparator.It all seems to work well and even a signal at 20 GHz is easily down converted (harmonic number 38...)Here is the question though.How are frequencies for LO1 and LO2 calculated?I have tracked the LO frequencies for an RF from 3.000025 to 3.001975 GHz with a step size of 50 kHz. There is no obvious structure in how LO1 and LO2 are chosen. Sometimes LO1 (first stage harmonic mixer) is incremented, sometimes it does not change and sometimes it is even decremented. Step size is always multiples of 50 kHz since that is the granularity of LO1 PLL. The change in LO1 is however not just 50 kHz, but can be even 450 kHz.Anyone with experience in how these frequencies are calculated? They are not random, but it appears to be quite complex.Is it related to rejecting possible harmonic signals in the down conversion chain?Is it related to optimizing the SRD input (not likely since the frequency variation is very small)?The RF source is a YIG oscillator where coarse frequency is set using a DAC. Fine tuning (driven by the PLL and a second DAC for initial search) is roughly +-25 MHz. Can it be that LO1 and LO2 are chosen so that not the wrong harmonic number shall generate a lock during the frequency search? Not clear why - LO1 is very clean although LO2 contains many harmonics.It should perhaps be noted that LO1 can be tuned in the range 357-536.5 MHz and LO2 can be tuned in the range 25-272.5 MHz, but only the ranges 520 - 535 MHz and 80 - 90 MHz are used once harmonics allow these.Regards,Staffan

author: virtualparticles
author_rank: Regular Contributor
timestamp: January 07, 2020, 04:08:49 pm
content: The LO frequencies are chosen to put the IF at the desired frequency of course but intermodulation products are always generated as well. These IM spurs are not a problem for a VNA as long as they appear outside the IF Bandwidth of the measurement They are filtered out by the DSP. At certain frequencies though, spurs will enter the IF bandwidth and there are two choices at that point. Either switch the LO to the other side of the RF or move the IF frequency slightly to get away from the spur. The LO can be either above or below the RF and generate the same IF frequency so it is possible to simply switch it back and forth as needed.The IF is either LO - RF or RF - LO depending on which side the LO is on.The IMs are generated by 3*LO - 2*RF, 4*LO - 3*RF, 5*LO - 4*RF and so on for low side LO and3*RF - 2*LO, 4*RF - 3*LO, 5*RF - 4*LO and so on for high side LO.So the spurs can be calculated but in practice, the IMs are often evaluated empirically and clever software determines how to move the LO to compensate.Cheers,Brian

author: Staffan321
author_rank: Regular Contributor
timestamp: January 08, 2020, 07:31:07 pm
content: Hello,Thanks. Yes, I'm familiar with the generic idea of selecting a suitable frequency. Problem here is that it seems like the 'clever' part in the algorithm is quite complex. First LO is stepping up and down in a quite "unpredictable way". Would be great to know if there is a document discussing this more in detail when it comes to a two stage down conversion like here, but I guess this is very much related to the very design and perhaps frequency selection algorithm has evolved through trial and error to get any frequency to work well? First mixing is always LSB and it seems like second mixing always is USB although LSB does give the same response.The VNA is a Wiltron 37247A by the way - anyone having reverse engineered the code (fingers crossed...)?Regards,Staffan

author: virtualparticles
author_rank: Regular Contributor
timestamp: June 15, 2020, 12:52:23 am
content: This is really a number theory problem and I imagine there are experts that could give better answers. When I designed Super-het systems, I wrote a brute force program to work it out. Not terribly clever, but it worked."Brute force and ignorance can overcome any obstacle, no matter how sophisticated."