Need the insight of a RF engineer - Page 1
Forum section: electronics / beginners

author: notaroketscientist
author_rank: Unknown
timestamp: May 09, 2015, 09:30:23 pm
content: I am trying to understand the criteria for rf mixing schemes applied to transverter use.Specifically I have transverter for 144-146mhz operation using a transceiver of 28mhz to 30mhz.The transverter uses an overtone crystal for a local oscillator frequency of 116mhz.The goal is to cover 144-148mhz with the least amount of fuss and the best quality signal.The transceiver being used as an IF is capable with slight modification of producing 26-32mhz output.Obviously the simplest approach is extending the output to 32mhz.Some of the questions which come to mind:Transverter input/output tuned circuits might need modification.Carrier operated transmit/receive switching could be frequency sensitive?Studying the schematic or measuring the frequency response of those circuits should offer answers BUTwhat other considerations exist which I have naively overlooked?Practically speaking are the tuned circuits involved likely to be broadbanded enough to function as is?Mostly I am interested in the mindset a designer would take and the general choices good practice and practical reality dictate.thanks for any insightjim

author: KJDS
author_rank: Super Contributor
timestamp: May 09, 2015, 10:30:58 pm
content: The first thing to consider is the frequency plan. Harmonics, and you'll probably have to consider up to the ninth, of both the 28-32Mhz transmitter and the 116 MHz oscillator will mix and if any of these is too close to the wanted frequency then you'll not be able to filter them out. The same will be true on the receive path.Then it's a case of sorting out levels, distortion and noise on both up and down paths to ensure that you have the sensitivity and selectivity and also don't transmit anything you shouldn't.Also consider all the filtering you'll need. In the UK then there are a lot of transmitters around 86MHz. Take that from 116 and you'll get 30MHz. There's probably at least a week of work just to get a scheme that I'd be prepared to consider starting to build. If I were considering the system design for something that would be going into production, rather than a one off, I'd probably want a month of solid work to make sure I was happy with all the compromises.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2015, 03:58:41 am
content: Extending the BW is probably theoretically achievable, though the extent required for the transceiver may not be simple (e.g., if its local oscillator is PLL synthesized, you have to hack the digital counter itself to even be able to reach those frequencies).There is a fundamental limit to the bandwidth of any stage: for a given system impedance Zo, the shunt capacitance (e.g., transistor collector / drain capacitance) must be less than 1 / (2*pi*Zo*BW). This is true, more or less independently of the center frequency. If baseband (i.e., a "wideband" amplifier, where the low frequency cutoff is many times below the high frequency cutoff, or all the way to DC), then this sets the high frequency cutoff (in essence, the center frequency is 0). For a tuned amplifier, the center frequency can be arbitrarily high, but BW remains more or less constant, and therefore the BW% drops as center frequency rises. (So a transistor with some constant capacitance can do DC to, say, 50MHz, or 100-150MHz, or 500-550MHz, but only each range at a time, in a suitably designed circuit.)Going from this theorem to actual circuit changes isn't simple. Inter-stage impedances are often higher (giving a better match to base/gate input impedances, and increasing gain, at the expense of bandwidth), or much much lower for power stages (big transistors require big currents..). Rather than carrying signals in transmission lines (of characteristic impedance), signals may be carried by coupling capacitors, transformers, matching networks or filters, all of which have a broad range of impedances (and component value interactions!).If the circuit is quite simple, few of these may apply, and the change might be pretty simple. But it still won't be as easy as plugging in proportionally different inductor and capacitor values. So it's really hard to say.Tim

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: May 10, 2015, 04:25:24 am
content: Quote from: notaroketscientist on May 09, 2015, 09:30:23 pmThe tuned circuits at 144 MHz should be OK, particularly the transmitter low pass filter.You may lose a bit of rx sensitivity and tx power output at either end of the band but that is all.If there is a sharp 28 - 32 MHz tuned circuit then that might require more work. Maybe tune it to 32 MHz and add varactor (or other diodes) to allow some variability down to 28 MHz?While it's nice covering 144 - 148 MHz, you need to consider if you really need all that range. If you're into CW/SSB (and that's what most transverter users use them for) then you'll only need to cover 144 - 145 MHz (or less) and there will be no problems with tuned circuit responses. But you will need to have other than carrier operated t/r switching as that won't work on SSB (there being no carrier).If you want FM your 28 MHz radio also need to have a 600 kHz offset for the repeaters and possibly CTCSS subtones. You may be able to obtain the offset with dual VFOs and/or memory channels. A ready-made FM rig will have all those features.Given that FM gear is cheap it might be better making the transverter into a high-performance unit dedicated to CW/SSB and buying a cheap FM mobile transceiver for FM & repeaters. Then there will be fewer hassles in getting optimum performance through its entire frequency range.

author: notaroketscientist
author_rank: Unknown
timestamp: May 10, 2015, 04:57:33 am
content: Quote from: KJDS on May 09, 2015, 10:30:58 pmThank you for the thoughtful reply. The actual mixing scheme is dependent upon the transceiver and the transverter only allowing for me to choose changing the local oscillator in the transverter and choosing the range of the transceiver. The possibilities of even those limited variables results in a matrix I am at a loss to analyze. Most likely once I am confident the transceiver isn't likely to fry the transverter I will test the combination by observing the "birdies" into a dummy load for receive and then with an antenna for images. The transmission spurs I can check with a spectrum analyzer.What approach do you use to predict the various mixing combinations?

author: notaroketscientist
author_rank: Unknown
timestamp: May 10, 2015, 05:01:45 am
content: Quote from: T3sl4co1l on May 10, 2015, 03:58:41 amThanks Tim I have never been exposed this equation. Where can I learn more? I got the gist but it is food for a great deal of cogitation.

author: notaroketscientist
author_rank: Unknown
timestamp: May 10, 2015, 05:29:00 am
content: Quote from: vk3yedotcom on May 10, 2015, 04:25:24 amThis is close to my present understanding. Nice to have some reassurance about the low pass filter. The input to the transverter and attenuation pad could have problems with going above 30mhz. The reference to COR is my parroting the common explanation for t/r switching without a separate PTT line. I believe it only goes to transmit when the applied audio drives the output and uses a delay to hold the transverter in transmit mode between syllables.Mostly the additional frequency coverage will allow monitoring of the FM portion of the band easily.I am using an IC-703 which has FM mode and CTSS tone encoding as well as split operation which can all be preserved in memory. The Icom is a digitally controlled radio and as such only requires the removal of a single diode to allow transmission outside the amateur bands. A spectrum analyzer will confirm a clean output.The stock frequency response of the Ten Tec transverter covers the SSB portion of the band but I can never let well enough alone. I have been "taking it apat" instead of turning it on for a long time and my imagination sends me to experiment with things just because I can. My curiosity has come at the price of destroying a lot of stuff over the years which in turn is a learning opportunity but this radio is dear to me so it seems prudent to consult with experts prior to testing for smoke.Thanks for your input.Jim

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 10, 2015, 09:47:11 am
content: Hi Jim,I'd say you are overthinking this a bit. There is no need to modify anything to find out how well it performs receiving 146-148 MHz since the transverter will happily do that already and the IC 703 is a receiver up to 60MHz. Maybe it will work with reduced specs but just try and find out, nothing will break!Then for the transmit side, there is no need to modify the transverter to do some tests, only the diode in the Icom. So, experiment and report backPaul

author: notaroketscientist
author_rank: Unknown
timestamp: May 10, 2015, 12:48:05 pm
content: Quote from: PA0PBZ on May 10, 2015, 09:47:11 amThanks Paul. You are on the mark. I am overthinking this intentionally to learn as much as I can from simple stuff. I ask the questions as much to test my understanding as any other reason.

author: KJDS
author_rank: Super Contributor
timestamp: May 10, 2015, 02:12:55 pm
content: It may not cause real problems, but (2*116)-(3*29)=145and (3*116)-(7*29)=145So around 145MHz there could be a lot more than desired coming out of the mixer. The first one I'd be very concerned with, you'll probably need to check on a decent spectrum analyzer that wasn't being transmitted at any appreciable power level. You won't see it when transmitting at exactly 145MHz as the wanted signal will be over the top, but go off by a few kHz and the sprog will be just the other side of 145MHz. If the nearest ham station is miles away, then it's far less of an issue than if you live in a city and have people close by.

author: notaroketscientist
author_rank: Unknown
timestamp: May 10, 2015, 02:55:54 pm
content: Quote from: KJDS on May 10, 2015, 02:12:55 pmGreat example. Is there a formula or some other tool which can make an analysis more easily than brute force?It can't be cost effective and it seems overwhelming to me.My experience lies is running the gear and looking for spurs or images not exactly the best tool for design and development.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 10, 2015, 03:18:48 pm
content: Quote from: notaroketscientist on May 10, 2015, 02:55:54 pmNot all can be done by formula and tool, it is very hard to predict exactly what the strength of the harmonics exactly are and what unintended interaction you will have between them. That is why after design the thing needs to be tested, and if it's bad redesigned until the results are satisfactory.Since this is a commercial transverter you should suspect it to be free of spurious like KJDS mentioned, but there is only one way to be sure!

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: May 10, 2015, 09:06:57 pm
content: Quote from: notaroketscientist on May 10, 2015, 02:55:54 pmI have seen charts that show this.From memory they have bars showing the various frequency ranges of fundamentals and harmonics of the component frequencies used. For instance fundamentals and harmonics of 28 - 32 MHz and 116 MHz (or lower assuming you're using a lower frequency crystal and multiplying). Try plotting on graph paper (X axis frequency, Y axis maybe order of harmonic - ie fundamental, x 2, x 3, x 4 etc. And / or Excel calculations showing various sums and differences of frequencies involved.Also the image of the tuning range (ie desired might be 28 to 32 + 116 = 144 to 148, whereas difference would be 116 - 28 to 32 = 88 to 84). 116 MHz being your mixing frequency. So if you had a very strong FM broadcast station on 88 MHz you could hear products of it when tuned to around 144 MHz if filtering is poor. Because of FM's wide bandwidth the products may be audible over the first few hundred kHz of the band.Plus sums and differences of harmonics of both the 28 MHz transmit frequency and the 116 MHz mixer frequency can produce birdies. But don't assume the 116 MHz if pure either if you're using a lower frequency crystal and multiplying as a stray harmonic of this can beat with 28 - 32 MHz to produce spurii on transmit and reception of undesired frequencies on receive.Where mixing of harmonics is involved you can get birdies that tune faster than desired signals.Moving the IF and mixing frequency means a whole new set of birdies and potential problems. Eg a 130 MHz mixing frequency allows a 14 - 18 MHz IF. However its image becomes half as far from the desired frequency and becomes harder to filter (in this case 130 - 14 to 18 = 116 to 112 MHz which is in the aircraft band.

author: notaroketscientist
author_rank: Unknown
timestamp: May 13, 2015, 09:57:45 am
content: Quote from: vk3yedotcom on May 10, 2015, 09:06:57 pmThanks for that post it is what I was looking for. Simple enough to layout the internal frequencies with harmonics and then superimpose a spectrogram of the local area. Makes for a starting point to visual and plan from.

author: G0HZU
author_rank: Super Contributor
timestamp: May 13, 2015, 03:43:37 pm
content: I designed a lot of RF up/down converters in the 1990s and I think I only used the old school (manual) mixer charts a couple of times.It's worth playing with them as a learning tool but ultimately, the most powerful way to do the analysis is to write your own software tools or buy a commercial program.Back in those days there were various programs available and I still use some of them today. The program screenshots below are from MSDOS SW that is 20 years old but it is still quite powerful. I had to use a camera to grab the screenshots as there is no easy way to grab a screen image to the clipboard on this old SW.You can see that one tool (Mccts) does a static analysis/prediction and the SynergyMW (better?) tool does a swept analysis. The level of the spurious responses is mixer drive dependent so the levels will change if you tell the software a different RF drive level or LO drive level. But these are only meant as a rough guide. The SW tools come with a library of mixers to try and improve the prediction but the results will still be affected by mixer termination impedances across the frequency band.I spent many MANY hours with these tools in the 1990sToday (at work) I use Agilent Genesys to do this type of analysis and it can deal with multi conversion up/down converters. Very powerful and very expensive! I also wrote my own SW tools to do similar and to also predict mixer spurious terms caused with multi conversion superhet designs.Note: I can't distribute any of this SW on here because it is/was purchased with a licence etc. But it wouldn't be that difficult to replicate the tools below if you included the correct equations for the various distortion terms wrt order and drive level. I don't think these old DOS tools are available today as they both went obsolete maybe 15 years ago. But I suppose you could ask Synergy or MiniCircuits?Note that I used a higher input drive level to the mixer for the Synergy plot (-5dBm compared to -10dBm) so the mixer spurious terms are quite a bit higher in level when compared to the MiniCircuits analysis.

author: notaroketscientist
author_rank: Unknown
timestamp: May 13, 2015, 04:06:38 pm
content: Bingo we have a winner! Thank you G0HZ. I knew there had to be some better way. I will search for the old software with which to play. Mostly for hobby use we plug it up and see what smokes and that works well enough but it didn't seem an efficient approach for well funded commercial endeavor needing more predictable results.Thank you so much for spending the time to point me in the correct direction. I owe you a beverage of your choosing.

author: G0HZU
author_rank: Super Contributor
timestamp: May 13, 2015, 04:29:46 pm
content: See attached doc file from 20 years ago that shows the instruction for Synergy MW Synspur and Locus...Locus is the tool to use as it does a swept analysis as per my previous screenshot. Synergy should still have this old SW 'somewhere' and I don't think it was very expensive back then. It was probably <£50 and is only about 100kb in size. If they still have it I think they may just give it to you free...I also had a look at your Ten Tec transverter circuit. It uses a crude homebrew double balanced mixer using 1N4148 diodes which won't give spectacular performance but it will still be adequate. The thing that concerned me the most was the design of the drive ALC. It appears to use a lossy/reflective attenuator with a shunt PIN diode. This will look like a small shunt capacitor of a few pF to your Icom driver radio (the PIN diode will appear resistive but the series 5pF capacitor at the input will dominate) and this is why there is a big 50R shunt resistor at the input here. It will presumably be rated at 20Watts or more and it also ensures a good termination to give low input VSWR despite the poor VSWR of the attenuator itself.Also, this PIN diode based shunt attenuator will generate distortion/harmonics and you can see that the 28-32MHz BPF that follows it has a zero (trap) in the design at 60MHz as L4=1uH and C13=5pF in parallel with 2pF in L4 is resonant at 60MHz. This will put a fairly deep null in the BPF response at the second harmonic at 56-64MHz. I think this zero/null is there to remove harmonic distortion introduced by the PIN attenuator diode plus any harmonics from the driver radio. This will therefore help a bit with mixer spurious terms in the upconversion.

author: notaroketscientist
author_rank: Unknown
timestamp: May 13, 2015, 04:52:10 pm
content: Quote from: G0HZU on May 13, 2015, 04:29:46 pmOkay now I owe you a beverage and a meal. Thank you for the critical appraisal of the device. I saw the 4148 mixer and wondered if there might be something better? I have several MiniCircuits devices which I believe would cover the frequencies. How would I best determine the i/o impedance of the 4148 based mixer? Gotta go back and look at the pin diode to see what purpose it serves. If it is t/r switching maybe a relay would be a better alternative.I realize this is all impractical as simply pulling the transmit frequency diode in the transceiver will likely result in acceptable function but it isn't the destination so much as the journey. Understanding this device well enough to modify it and improve it in this manner increments my comprehension of the principles without slamming my face into the brick wall of math. Harry Callahan (Dirty Harry) said a man has got to know his limitations. I am all too familiar with mine. The math makes much more sense with an empirical demonstration. Learning to fish for dinner is eminently more satisfying than eating a donated fish.

author: G0HZU
author_rank: Super Contributor
timestamp: May 13, 2015, 05:05:47 pm
content: The mixer is probably fine...I can give you another old school rule of thumb for the mixer drive level if that helps?If you feed a mixer with the classic two tone test signal then (in your case) I think you would want to keep the 3rd order distortion (IMD3) products below -40dBc. By the PA stage they will degrade to maybe -26dBc but they need to be better than this at the mixer.So I suspect that TenTEc will have designed the ALC to achieve the following (ballpark)The 3rd order terms will beIMDdBc = 2*(MixerIP3 - (Input tone Level))A typical level 7 mixer will have an input MixerIP3 of +15dBm so you would want to keep the 28MHz test tone levels at -5dBm or lower becauseIMDdBc = 2*( 15 - (-5)) = -40dBcyou can see from the Locus plot that just one -5dBm drive tone can give other spurious terms at ~ -50dBc so for best performance you would want to drive it slightly less than this at 28MHz to give the spurious plot below.But TenTec's ALC circuit will currently (automatically) determine this mixer drive for you anyway because it will auto adjust the drive level peaks to prevent mixer overdrive.

author: notaroketscientist
author_rank: Unknown
timestamp: May 13, 2015, 05:28:54 pm
content: This is real bread. Thanks. I am designing a homebrew hf receiver and this type of insight into mixers is very helpful.

author: G0HZU
author_rank: Super Contributor
timestamp: May 13, 2015, 05:36:46 pm
content: Note that the above IMD approximations are only valid if you terminate the mixer port in a broadband 50R impedance. Many designs place an RF filter directly after the mixer and this will cause reflections from some (unwanted) mixer terms and this usually degrades the distortion performance.But if you look at TenTec's design they have been quite clever and they have used a J310 JFET (Q6) in common gate mode directly after the mixer in the transmit path.This will have a broadband input impedance of 1/gm = 1/0.013 = 77 Ohms approx. This is going to be close enough to 50 Ohm to give very good performanceSo my guess is they designed the ALC to give a drive level similar to my equations above. If you drive it much harder than this then the IMD3 distortion terms and the other mixer terms in the Locus analysis will shoot up to much higher levels...So TenTec's ALC system will let you drive this transverter with maybe 20W PEP and the ALC will auto adjust the PIN diode attenuator at the input to always attenuate the input signal to keep the mixer drive level within the required limits.Note: The PIN diode in the attenuator doesn't have to cope with 20W because the attenuator is designed to be highly reflective. So the input signal is fed to the PIN diode via a tiny capacitor. So although the input signal from the drive radio could be huge in terms of Vrms, the PIN diode will only see a much smaller signal. But I do think it will still introduce some distortion of its own so that is probably why there is a 60MHz trap in the BPF after the attenuator. This will prevent harmonic distortion from the attenuator from reaching the mixer.

author: notaroketscientist
author_rank: Unknown
timestamp: May 13, 2015, 05:49:52 pm
content: Got it. You have been immeasurably helpful.

author: G0HZU
author_rank: Super Contributor
timestamp: May 13, 2015, 06:25:56 pm
content: That's OKI'm a bit bored waiting for my meal to cook so I fired up a few sig gens here and connected them to a MiniCircuits (DB) mixer and fed this to a E4406A analyser.The two IF tones are at 29.050MHz and 29.070MHz and they are fed into the mixer at -5dBm each. The LO signal is 116MHz.You can see the mixer output spectrum in the plot below. The mixer has about 7dB loss so each tone exits the mixer at -12dBm at approx 145MHz. Have a go at analysing the spectrum and see if you can identify what n*RF +/- m*LO is causing each cluster of spurious terms on the low side.You can see that the IMD3 terms are at about -40dBc as predicted by the classic equations I posted up earlier. But see if you can identify why there are other clusters of terms there as well.Note: I haven't attempted to analyse it myself as my meal is nearly readyThe mixer in your TenTec transverter will probably give very similar performance to this plot but it may be slightly degraded due to the use of 1N4148 diodes.

author: G0HZU
author_rank: Super Contributor
timestamp: May 13, 2015, 06:49:41 pm
content: You may find the two tone spectrum a little confusing to look at. So if you need some more info, then see below for a single tone spurious prediction for 29.050MHz into a Minicircuits mixer at a slightly lower drive level.The terms are there but lower in level because the mixer tone level is -10dBm rather than -5dBm. Also there are far fewer terms in the plot with a single tone test signal.But the plot below gives the LO RF orders in the top right cornerNote: I've used MiniCircuits' design tool here rather than Synergy.See also a plot of just one tone into the mixer on my test bench. The two agree quite well although the 'real' mixer is being driven slightly harder so the terms will be higher.

author: notaroketscientist
author_rank: Unknown
timestamp: May 13, 2015, 06:59:37 pm
content: Well I don't grasp why there are spurs every 200khz unless it is related to the 50hz mains frequency in the UK but I would expect there to be more spurs? They are very low level. My ignorance is displayed.

author: G0HZU
author_rank: Super Contributor
timestamp: May 13, 2015, 07:08:13 pm
content: The two tone test signal is probably going to be too confusing to work with so just look at the (single tone) simulation and the measured plot in my previous plots.You can see the main wanted signal but you can also see the tone just to the left of it. This is the 3RF 2LO mixer term KJDS warned you about way back in post #9.(29.050 *3) - (116 *2 ) = 144.850MHz. So you can see that there is an unwanted (distortion) tone at this frequency in the plotIt is low enough to not really worry about in this case. if it was -40dBc then I guess it would become a bit naughty to transmit a spurious like this but as long as it is lower than -45dBc then there isn't a problem. In the measured plot above it is -55dBc with -5dBm drive level.

author: notaroketscientist
author_rank: Unknown
timestamp: May 13, 2015, 07:17:19 pm
content: So one last dumb question then I gotta hit the books:what is the best approach to reduce those spurs?

author: G0HZU
author_rank: Super Contributor
timestamp: May 13, 2015, 07:23:20 pm
content: Assuming you want to keep the same frequency plan then you can either reduce the mixer drive level and/or try running a mixer that needs a higher LO drive level. eg level 10 or level 13 (or higher).But there really is no need if your transverter already achieves -55dBc as per my plot.Note if you scroll down to table 2 in the review link below, you can see that the claimed spurious spec is -56dBc with -58dBc measured. I would expect that the spurious they measured is this 3RF 2LO term.http://hamradio.online.ru/ftp/pr0006.pdfIt really isn't a problem. You may find that your Icom radio that drives the transverter will add a few spurious terms of its own and they may be even bigger than this! But it really isn't going to be an issue in the real world

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 13, 2015, 08:02:21 pm
content: Quote from: G0HZU on May 13, 2015, 06:25:56 pmI fail to see anything above -55dBc in the plot, what am I doing wrong?

author: KJDS
author_rank: Super Contributor
timestamp: May 13, 2015, 08:14:35 pm
content: Quote from: PA0PBZ on May 13, 2015, 08:02:21 pmThe IMD3 terms are the ones close to the two wanted carriers. The ones further out at -55dBc are IMD5, they come from the 3LO-2RF, 3+2=5 so it's IMD5. The close in ones are a 2-1 product, and 2+1 is 3 so are the !MD3 products.Hope that's a simple enough explanation.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 13, 2015, 08:16:14 pm
content: Quote from: KJDS on May 13, 2015, 08:14:35 pmYes, thank you, it's clear now.

author: G0HZU
author_rank: Super Contributor
timestamp: May 13, 2015, 10:52:01 pm
content: In case anyone else is still a bit unsure then the -40dBc relationship for the IMD3 terms then it is shown in the marked up image below. This level of (distortion) performance is predicted with the classic equation in post #18 when the mixer drive tones are at -5dBm each. The mixer has 7dB conversion loss so the plot below shows the two main (wanted) test tones leaving the mixer at -12dBm each in the analyser plot below. The two (unwanted) IMD3 tones are about 40dB below this level as predicted byt the equationAlso, I pasted together a schematic of the tenTec 1210 transverter from a German review of this transverter. You can see the transmit signal path from the input, through the attenuator and the 28-32MHz BPF, the mixer and then to the J310 JFET Q6.I've circled PIN diode D8 in the input attenuator and also the L4/C13 trap in the 28-32MHz BPF and also the J310 JFET Q6.