put AM through it and see how much you get out... that is a limiter test.....
I use the analog devices tool ADI sim RF. excellent. I think worrying about mixer terminations is important but you have bigger fish to fry yet. Try ADL5350 for a mixer.watch spurious products. g On 29/10/2015 12:31 PM, David Rowe wrote: > Hi Guys, > > Thanks again for all your tips and insight. I'm following up yr > references and doing lots of reading. > > Glen - I haven't decided if I'll undersample yet or use a 2nd mixer from > 10.7MHz to low IF. > > Steve - I have built up a spreadsheet to work out cascaded NF, MDS, and > ADC NF. Am working through each calculation step nice and slowly! Also > prototyping some of this and getting results a few dB from my > calculations which is encouraging. > > Matt I'm using a termination insensitive amp at the mixer IF port, have > measured it's return loss as 20dB between 10.7MHz and 300MHz (2LO is > 272MHz). > > Glen - what problems should I look out for with a limiting amp? Any > tests I can do to spot issues early? I guess two-tone wouldn't work. > > Cheers, > > David > > On 29/10/15 10:28, glen english wrote: >> Hi David >> Glad my description of NF made sense. >> >> If your limiter is perfectly doing its job, then yes 1 bit is all you need. >> >> I like seeing the xtal filter in there ! This reduces the umming and >> ahhhing by two to three orders of magnitude. >> >> Of course the filter stop-bands are finite, so you must consider this, also. >> >> I'm guessing you are going to undersample. Goes without saying that ADC >> performance degrades quickly on super nyquist but this is unlikely to be >> any issue in this design. >> >> Final sample rate needs to be at least two times the IF filter BW, 4x if >> you want to not make life hell for your digital filters. >> >> My guess is, you could use a single pin of the STM32 inconjunction with >> some timer and input compare block to get a very nice and periodic 1 >> bit sampler. >> >> Dont expect miracles though. Analog limiters are dreadful things. if you >> want some REALLY good limiter lessons tuition , go look at the FM >> limiter in a analog VIDEO CASSETTE RECORDER. They are the very best, >> high bandwidth (10 MHz) limiters around.... >> >> For a IF width of 15kHz, I'd suggest a SR of about 60 k. ( an enormous >> undersample) ..That will help also preventing strong adjacent channels >> aliasing. >> >> regards >> >> >> >> >> >> >> >> >> >> On 29/10/2015 8:00 AM, David Rowe wrote: >>> Thank you Helmut, Glen, and Steve, >>> >>> 1/ Ok it's starting to make sense, lets see if I can work through a >>> contrived example: >>> >>> We have a 16 bit sound blaster ADC, with a SNR of 6*15 = 90dB. Its FSD >>> is 1Vrms (10dBm). It samples at 48 kHz: >>> >>> The ADC noise floor is at 10-96=-86dBm. This is in a Nyquist BW of >>> 48kHz, so the noise power normalised to a 1Hz BW is -86-10*log10(24E3) = >>> -129.8dBm/Hz. The thermal noise floor is -174dBm/Hz so our ADC NF is >>> 174-129.8 = 44.2dB. >>> >>> Help me understand what NF is too, and why a filter has a negative NF - >>> it moves the signal closer to the thermal noise floor. >>> >>> 2/ I'm actually working on narrow band constant envelope radio, So: >>> >>> BPF- LNA - Mixer - Xtal filter - limiting amp - ADC >>> >>> If I am sampling a constant envelope signal (FM, FSK, GMSK), what are >>> the SNR/SFDR requirements for the ADC? The limiting amp has removed all >>> amplitude information. So do we just need the sign bit of the ADC? >>> Could we just sample the signal with a flip flop? >>> >>> Thanks, >>> >>> David >>> >>> >>> On 28/10/15 17:03, glen english wrote: >>>> Howdy >>>> A good response Helmut >>>> >>>> The oversampling ratio (OSR) , clock purity will dominate. >>>> >>>> What is your planned OSR , and sampler rate ? >>>> quadruple the sample rate , gain a bit of ADC of course. >>>> >>>> With an OSR = 1, for -120dBm , and full scale of say +12dBm (roughly >>>> what you have) , and 60dB of SNR will put your noise floor at 12-60 = >>>> -48dBm, so you'll need 72dB of gain. And so it will overload on the >>>> slightest thing out there ! >>>> >>>> With an OSR of 4x, you are 6dB or 1 bit better off, and so on. I know >>>> you understand this stuff so I wont elaborate. >>>> >>>> The SFDR of the converter will dominate what it can USEFULLY hear, >>>> because below the SFDR , REGARDLESS of the OSR, there will be all sorts >>>> of funny converter artifacts, and the intermods will be there also. >>>> So the SFDR , not the OSR ultimately determines the performance capability. >>>> >>>> IE the SNR might be 60dB, say a 10 bit converter, >>>> if the OSR = 4 then you'll get 66dB SNR, BUTthe SFDR does not change, >>>> the SFDR is still 60dB. So you can improve dynamic range, but not the >>>> SFDR. The SFDR, or more likely, where the third order two tone intermods >>>> are, won't change. Some LT converters have incredibly good SFDRs via >>>> internal digital dithering (later subtracted out in your receiver) . >>>> >>>> For my commercial SDR, I use a 12 bit converter at 200 Msps. >>>> The SFDR is 96dB, approx. >>>> >>>> The converter input FSD is abotu +12dBm, so the IMD will be always 12-96 >>>> = -84dBm >>>> >>>> so if I want my IMD down at -120dBm, then I need 36dB gain in front of >>>> the receiver. >>>> With such a high OSR (200M/ 10k)=43dB , the SNR is off in the >>>> stratosphere, but the IMD dominates.... >>>> >>>> In my experience the SFDR is what will limit the sensitivity. >>>> >>>> Watch out for ALIASED noise also. don't forget your converter is also >>>> equally (almost) bringing in noise form 2fs, 3fs 4fs etc. SO important >>>> that the convertor is seeing a low pass (nyquist) or band pass filter >>>> (super nyquist sampling) >>>> >>>> NOW what you can do is vary the voltage that the converter sees by >>>> fiddlign with the termination and the nosie figure can be usefully >>>> manipulated +/- 12dB (improved at the expense of full scale level) >>>> >>>> The noise figure of the converter is approx (the input level - the SNR) >>>> - 174 >>>> >>>> IE +12dBm FSD, SNR = 70, noise floor = -58dBm. >>>> Now, that is for a 200 Msps, or 100 Msps nyquist bandwidth, that is >>>> 10log10(1e8) or 80dB >>>> so -58 - 80 = -138dBm dBcHzSNR or 174 - 138 = 36dB. >>>> >>>> >>>> cheers >>>> >>>> >>>> >>>> >>>> >>>> >>>> On 28/10/2015 6:47 AM, David Rowe wrote: >>>>> Hello Glen/Matt, >>>>> >>>>> I'm working on a VHF radio prototype for testing some of my open source >>>>> DV ideas. Could you pls explain how to work out the gain required in >>>>> front of my ADC? >>>>> >>>>> For example if I have a MDS of -120dBm (0.224uV), and an ADC with a 3Vpp >>>>> (1.06Vrms) clipping point, and SFDR of say 60dB. >>>>> >>>>> Is the gain rqd simply Av=1.06/0.224E-16? That would mean the minimum >>>>> signal would hit full scale on the ADC. Perhaps we could scale that >>>>> back by 60dB plus some margin such that the MDS is still a few dB above >>>>> the floor of the ADC. >>>>> >>>>> I'm a bit mixed up by the idea of NF and ADCs. A worked example would >>>>> help. >>>>> >>>>> Anyone else on the list with receiver design skills, pls feel free to >>>>> comment. If a good reference exists I'm happy to dig that up. >>>>> >>>>> Thanks, >>>>> >>>>> David >>>>> >>>>> ------------------------------------------------------------------------------ >>>>> _______________________________________________ >>>>> Freetel-codec2 mailing list >>>>> [email protected] >>>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>>>> >>> ------------------------------------------------------------------------------ >>> _______________________________________________ >>> Freetel-codec2 mailing list >>> [email protected] >>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> > ------------------------------------------------------------------------------ > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > -- - Glen English RF Communications and Electronics Engineer CORTEX RF & Pacific Media Technologies Pty Ltd ABN 40 075 532 008 PO Box 5231 Lyneham ACT 2602, Australia. au mobile : +61 (0)418 975077 ------------------------------------------------------------------------------ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
