Design a HF to VHF upconverter for a spectrum analyzer (CMU200) - Page 1
Forum section: electronics / rf-microwave

author: Yansi
author_rank: Super Contributor
timestamp: December 10, 2020, 06:47:21 pm
content: Hello,just need to relieve some steam from the usual work and family related stress, need to cobble something together, so I can get some "soldering therapy" for Xmas, so I thought:I've recently acquired a CMU200, that can receive signals from 10 MHz plus, but does not work below 10 MHz. I though I could design a frequency up-converter and mix the "0" to 10 MHz to say 100 to 110 MHz band, where the CMU200 is operational.The basic idea is sketched below.The input signal gets low pass filtered and mixed with a 100 MHz LO. The low pass filter should have enough stop band rejection, so unwanted products do not interfere with the measurement. The low pass filter is accompanied by a pair of attenuators, both to improve impedance matching and attenuating as well.I though a sensible design goal would be 0 dBm input maximum, which is kind of on the high side for the ADE-1 mixer (15 dBm IP3). With both attenuators and the filter, I may get up to some 10dB of attenuation, so about -10 dBm at the mixer IF port (which should leave third order inter-modulation products 50 dB below the desired products, which I guess is good enough.The 100 MHz LO will be obtained using an integer-N PLL (ADF4360-8 comes into mind), will use a 10 MHz reference provided from the back of the CMU200, so we do not have the LO drifting around.To ease the use of the up-converter gizmo, it would be nice to have the output level match the input level. I guess designing the response very flat may be quite a challenge, but at least, we could slap a 15 dB amplifier on the RF output, to correct for the input filter loss and mixing loss (about 5 dB). However, this will also increase noise floor quite some, as we will be amplifying the attenuated signal.Also I think the IF port of the mixer should see 50 ohm impedance for as wide as possible frequency range, would it therefore make sense (any any measurable improvement in quality), if a diplexer would be added at the IF port? Basically a low pass branch to the input LP filter and a high pass branch to a 50 ohm termination.Could you please verify my thoughts are correct or possibly suggest improvements, what should be done differently and why? Is this approach correct, or am I way off?Regards,Y.

author: CJay
author_rank: Super Contributor
timestamp: December 10, 2020, 08:13:40 pm
content: Umm, just use one of the RF signal generator outputs on the CMU?

author: geggi1
author_rank: Frequent Contributor
timestamp: December 10, 2020, 08:41:19 pm
content: Look for a design used with a TV dongle SDR.Typically it consist of a mixer and a oscillator.With a SA612 or 602 its buildt with crystal.With DBM its built with a canned ttl oscillator typical a 100 Mhz.

author: Yansi
author_rank: Super Contributor
timestamp: December 10, 2020, 09:13:33 pm
content: Quote from: geggi1 on December 10, 2020, 08:41:19 pmLemme explain a bit:Yes, those converters are the typicalcheapdesigns, and SA602/612 is likely the most evil thing you could use there. Its IP3 of -15dBm is beyond hilarious. That is really a joke, to try use it for mixing high level signals. For reception from an antenna, maybe fine, but not really for an application where 0dBm input is the requirement.Canned oscillator - I have explained why it is not a good idea. You should have the LO locked to the analyzer's reference and make the PLL/VCO as clean as possible of phase noise.There are many reasons why just a mixer and oscillator is not enough. At least because many mirror frequencies will get mixed in to the 100-110 MHz output too, if nothing else. And mind you, there may be many of those present. You need to at least get rid of those on the input.Maybe even 100MHz LO is not the best thing to do anyway, due to the presence of strong RF interference from nearby FM broadcast transmitters. (However adding just 100 MHz is convenient for the user in the end).I do not want to use the generator source from the CMU, to leave it free for other tasks, not to mention the output to input isolation on the CMU is quite bad to begin with. Not that the ADE-1 mixer would have any kind of fancy LO to RF isolation, but at 100 MHz, it is about 60 dB. Is likely bit better, that what the CMU itself provides in between the output and its input.

author: CJay
author_rank: Super Contributor
timestamp: December 11, 2020, 11:47:12 am
content: Ah, ok, I get the need for higher separation..ISTR you can set a different ref output but that would suggest the ref output isn't direct from the oscillator so YMMV on the quality of it.

author: Yansi
author_rank: Super Contributor
timestamp: December 11, 2020, 01:58:10 pm
content: I am not trying to convince myself about my truth, just arguing, why I think cheap solution with SA612 and a canned oscillator will not work as supposed to.I am unsure if what I have drawn makes any sense, from an RF designer point of view - as obviously, I am no RF designer - a hobbyist at best.The low pass filter structure certainly is not 50 ohm anywhere outside its passband (either short or open, depending whether the filter structure begins with a series or a shunt element), so the 3.7 dB attenuator should help with that a bit. It should give about -7 dB S11 if I remember correctly. Without the attenuator, the S11 is 0 dB outside passband.The design would likely benefit from a full double-sided lowpass/highpass diplexer - that way the signal input will be close to 50 ohm from DC way up, and it should also make the mixer more happy. I will think about how that diplexer could be designed.You mean the selectable REF frequency out on the CMU200? I think that comes somewhere from the A7 digital module, not sure what generates it there and if it is locked to the main OCXO. But likely it is.

author: geggi1
author_rank: Frequent Contributor
timestamp: December 12, 2020, 08:29:58 pm
content: You should not use a SA612 /602 with a canned oscillator because this will give you a lot of harmonics.A canned oscillator can be used with a DBM because a DBM requres more drive than a gilbert cell mixer.Here is a good example of a DBM converter:https://www.on1bes.be/sdr_up_conv_v1.0_ade1_125_en.htmlI have seen examples os DBM converters that uses the LO input for the receiving port because this will normally go further down in frequency than the normal input port.If you are using a SA612 or similar you must use a crystal as the oscillator.

author: CJay
author_rank: Super Contributor
timestamp: December 12, 2020, 09:03:02 pm
content: Quote from: Yansi on December 11, 2020, 01:58:10 pmI'd be surprised if it wasn't locked to the main reference, it will be stable and it may be a better option than an external PLL if you set it for 25MHz then multiply and filter it.I'd hope it's nice and clean but if it's only provided to lock another instrument then it doesn't necessarily have to be so it may be a good idea to check.Agree the ADE-1 is a better choice than the NE612, they're bad on paper but it really depends what you're trying to measure/convert and how you implement.

author: geggi1
author_rank: Frequent Contributor
timestamp: December 14, 2020, 07:51:10 pm
content: https://www.giangrandi.org/electronics/lwupconv/lwupconv.shtmlHere is an good one.

author: Yansi
author_rank: Super Contributor
timestamp: December 16, 2020, 10:59:25 am
content: Quote from: CJay on December 12, 2020, 09:03:02 pmThats what I'm trying to discuss here. Input level requirement is 0dBm (for the entire converter-thingy), that is I guess well out of the range of NE612, even with all the input attenuation included.Quote from: geggi1 on December 14, 2020, 07:51:10 pmVery similar solution to mine. But I do not think I will need the LO cancellation on the output that much (interesting shunt crystal use right there).Just would be interested to hear if this kind of approach is correct, or if there is any better one (apart from getting a proper spectrum analyzer in the first place)Also if I'll be designing my own PLL, what should I look for, to not screw up too much?Thx, Y.

author: JohnPen
author_rank: Regular Contributor
timestamp: December 18, 2020, 11:54:11 am
content: Some very useful design information that may assist can be found at Scotties Spectrum Analyser see link below. Particularly with regard to Mixers and PLL generation.https://scottyspectrumanalyzer.us/JohnP