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Si5351 VFO - Low Pass?
¿ªÔÆÌåÓýSpurious responses caused by the odd-order harmonics inherent in a square-wave local oscillator are relatively easy to eliminate with front-end filtering. What is much more difficult to eliminate is the intermodulation distortion caused by having the mixer diodes only partially conducting for a significant proportion of a cycle - that's what happens if you use a sine-wave local oscillator.Steve G3TXQ |
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Whether a LPF at the VFO frequency lowers your noise level may or may not be noticeable, however the Si5351 outputs Square waves that are inherently high in harmonic energy which is why you need a LPF. The mixer should be mixing ONLY the signal we want and not all of the harmonics being generated, so, in the end you may not notice anything different with your ears, but I'm going to include a LPF because it's the proper way to do it and it's easy. Nobody is saying that it wont work without one. Also there are many commercially designed VFO's driving diode ring mixers with sine wave outputs that perform perfectly, I've built many of them including simple Collpits Crystal oscillators, they work perfectly. YMMV.
Joel KB6QVI |
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P.S. I meant to also mention, I agree with those who said that a squarewave is ideally better for a diode ring mixer. I think that theoretically the diode ring mixer should be more noisy with a sinewave drive, than squarewave. I think if there is noise its source is probably elsewhere... On Fri, Jan 6, 2017 at 11:25 PM, Hans Summers <hans.summers@...> wrote:
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I agree with Jerry I don't think any detectable extra noise will be caused by the Si5351A. If you have noise it is likely coming in from elsewhere - such as the processor for example. (By the way I have not heard of anyone reporting the QRP Labs VFO is noisy, I think the earlier comments about noise were from an SI5351A in a different system). It would be quite nice to have some more specific details, everything so far was a bit vague. Fact is, when you use the analogue VFO built on the BITX40 board, everything is close by. When you pipe in an external VFO then unless you are careful, you have introduced much longer wire lengths, and opportunity for ground loops etc. All of these can help pick up noise. As far as I can tell a lot of ground loop elimination is as much black magic as it is science. Easy things to try are to put an inductor in series with the supply voltage to digital circuits, with a decoupling capacitor either side. That's a Low Pass Filter and keeps RF out of the supply lines. A lot of times if there is interference from digital circuits, this helps a lot. I agree also, the 40m LPF is not the right one to use, because the VFO is at 5MHz not 7MHz. We have a 60m LPF which would be suitable .? 73 Hans G0UPL |
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I know I have RFI problems in the house, between the WiFi and the CFL lamps. So much so, that I can only reliably receive two or three of the local AM broadcast stations in the house whereas if I take a radio outside I can pick up a LOT more (and no, I don't live in a "Faraday cage - we have a wood framed house with vinyl siding; only the roof is metal, but the problem existed before the metal roof was installed). Rich KC8MWG On Friday, January 6, 2017 12:29 PM, "jgaffke@... [BITX20]" wrote:
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It's quite common for noise sources in the house to go up the coax and come back in from the antenna.
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Could be you need a good "common mode choke". ?? ?? My totally uninformed opinion: ?I think the Si5351 noise everybody's talking about is likely hash from the processor driving it. ?And that if this less-than-optimal LPF is reducing the noise to where it's as good as the analog VFO, that's good enough. ?The ideal would be to clean up the noise somehow without band specific filters, then we're back to switching only two filters if we want a multiband rig. ? The diode ring mixer wants to be driven hard. ?The ideal is for two of the diodes to turn on for 1/2 the LO cycle, the other two diodes to turn on for the other half of the cycle. ?Anything less, and changes in the other input signal (from the antenna when receiving) can disrupt the desired action of this commutating mixer. ?A square wave is actually preferred, and a perfectly clean sine wave from the LO will have a totally non-linear response from the diodes anyway since the current through a diode is exponential with voltage, not linear like a resistor. Of course I may well have totally different opinions once I get a Bitx to play with. Jerry ---In BITX20@..., <caulktel@...> wrote : If your using the QRPlabs VFO for the BitX40, then the LPF you bought for 40 meters is optimized for 7-7.5MHz. You need a LPF for the actual VFO output which is 4.5-5MHz. The 7MHz one is probably helping to clean the output somewhat, but is not optimal.? On Jan 5, 2017, at 9:53 PM,?dvoorhees@...?[BITX20] <BITX20@...> wrote:...I'm experiencing a high level local noise, but without the antenna it's very quiet.? ? |
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¿ªÔÆÌåÓýIf your using the QRPlabs VFO for the BitX40, then the LPF you bought for 40 meters is optimized for 7-7.5MHz. You need a LPF for the actual VFO output which is 4.5-5MHz. The 7MHz one is probably helping to clean the output somewhat, but is not optimal. Joel? KB6QVI On Jan 5, 2017, at 9:53 PM, dvoorhees@... [BITX20] <BITX20@...> wrote:
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When I bought the VFO kit from Qrp-Labs, I also bought a 40 meter low pass filter kit. I installed it in line with the output of the VFO. Pretty simple, The receiver seems quiet, at least as quiet as with the analog tuning. However I don't have a scope and currently I'm experiencing a high level local noise, but without the antenna it's very quiet.?
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Mvs Sarma
After all we have? band pass filter and crystal filter (for LSB /USB) after the mixer to take care of unwanted. we also have a Low pass filter to cut off harmonics possibly created by Lin PA. Best of 2017 sarma ?vu3zmv ? On Thursday, 5 January 2017 9:16 AM, "Baruch Atta baruchatta@... [BITX20]" wrote:
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I thought that a pure sine wave was best. Isn't a square wave a mix of waves?? That would result in all sorts birdies and products?
On Jan 4, 2017 10:28 PM, "Ken Chase chase8043@... [BITX20]" <BITX20@...> wrote:
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Just another thing to keep in mind, with the stock vfo 4.8 to 5.00 MHz, 30 times the vfo frequency will produce birdies in the 2 metre band, a case for using a metal enclosure, pardon the pun.
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Alf vk2yac iam74@... [BITX20] wrote:
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Hi all I have to add some comments here. 1) You cannot see or measure phase noise on an oscilloscope. You cannot even see or certainly, not accurately measure, phase noise on a spectrum analyser, unless it's a really excellent one. Phase noise is rather difficult to measure! 2) The phase noise of an Si5351A Synthesiser is certainly very much less than that of the analogue VCO/VFO in the BITX40 or any other analogue VFO. Yes the phase noise of an Si5351A is not as good as a crystal oscillator. But it is sure that if you replace the analogue VFO of your BITX40 with an Si5351A Synth or a DDS, and IF you notice an increase in noise, then that is NOT due to phase noise. There must be some other culprit.? 3) Gywn G3ZIL did an interesting and thorough comparison of the QRP Labs Si5351A Synth module with a crystal oscillator LO in a low noise direct conversion receiver. You can read his write-up here? . His conclusion was that the Si5351A noise was about 6dB worse than a well-designed crystal LO, and that 6dB would be of no consequence anyway relative to the background noise floor on HF. An analogue VFO would be much more than 6dB relative to a crystal. 4) The Si5351A libraries of Jason NT7S and Adafruit are both derived from the Linux Si5351A Driver. In my opinion what is fine for a powerful 32-bit processor is too bloated to use on an 8-bit system such as AVR, PIC and Arduinos. Furthermore the library caters for many possibilities, and also for the Si5351B and Si5351C variants, which makes it rather large and difficult to understand, particularly for non-expert coders. That's why I developed my own simplified Si5351A code. The QRP Labs website has several code examples, adapted for native AVR use, PIC, and Arduino environments. Of course these libraries work on the QRP Labs Synth module, or Jason's Etherkits Si5351A module, or Adafruit's module. See: 5) The QRP Labs Synth module differs from Adafruit and Etherkit modules in the following respects: ? ?a) It is a kit, using all through-hole components except for the Si5351A which is already soldered by the PCB factory. ? ?b) At $7.75 it is the lowest priced of these three modules ? ?c) It has a 2x 10-pin DIP pin header suitable for use in matrix board, breadboards etc. RF outputs are available on the header pins or you could fit SMA sockets.? ? ?d) There are pads for a 25MHz or 27MHz TCXO if you wish to replace the supplied 27MHz crystal with a TCXO ? ?e) The module was also designed to have the same footprint as the popular eBay AD9850 DDS module, and a somewhat similar pinout. The power pins match the AD9850 DDS, and the I2C bus pins are on the same pins as the AD9850 DDS module control pins. The 3 outputs of the Si5351A module are connected to header pins which match where the AD9850 DDS output some of its signals. Therefore in many applications the QRP Labs Si5351A module may be plugged in where an AD9850 DDS previously sat, and only the software needs to be changed to talk to the Si5351A. 73 Hans G0UPL |
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A diode ring mixer is inherently a non-linear device. Look at the voltage curves for diodes and you will see that there is a floor near the inverse voltage that guarantees that there will be never be instantaneous transition from positive to negative at zero volts (the ideal).
One uses diode ring mixers because they have an immense dynamic range compared to others. That being said, one must pay attention to the frequencies one feeds into the mixer. There will always be birdies. One must plan on them being outside of the bands being used so that they are not a problem. Figuring out mixer products to the 4th or 5th harmonic on a spreadsheet is very helpful. There will also always be some IMD. But planning proper impedance matching and filtering will always make it tolerable (i.e., down 40 dB or more). So it doesn't matter whether or not one feeds a square wave or sine wave. (The square wave is more or less preferred as it more easily saturates the diodes). There are inherent problems which must be addressed. The thing that really matters is the level of input signal and where are the gozintas and gozouttas. The RF must be clean and be capable of 140 dB or so gain level. The LO should be around 7 dbM. The IF must be impedance matched and able to take care of reflected waves. SWR is important; most of the noise and distortion products come from improper feedback from the IF; there have been all sorts of solutions to that problem (especially diplexers). Still, unless they are very noticeable, most ears can adjust fairly quickly to some level of noise and it is not really a problem -- especially with noisy amplifiers surrounding the mixers... The BITX solution is quite simple: provide a IF from the mixer that guarantees no in-band birdies, and provide broadband amplifiers around it which minimize reflection back into it. Along with a good ladder filter (the same is true there) It works fairly well. There have been some mods done, most prominently the LC filter on transmit of the BITXxxA series. You may find some to be necessary to conform to the transmit laws of your country. If I were to provide a filter on the DDS into the mixer, I would consider some sort of low Q? LC filter, which is really a variant of a diplexer. But it is probably overkill if the input level is right AND it is fed into the proper terminal. john AD5YE ---In BITX20@..., <jgaffke@...> wrote : With a diode ring mixer, those diodes are pretty much either conducting or not conducting. ?So the action of the mixer is a square wave, even if you feed it a nice clean sine wave. Given the balanced nature of the mixer, the narrow band crystal filter, and the band pass filter on the other side out toward the antenna, all those birdies should never come up. ?And these guys are talking broadband noise, not birdies.? Not that I know anything for sure here. ? Jerry ---In BITX20@..., <baruchatta@...> wrote : I thought that a pure sine wave was best. Isn't a square wave a mix of waves?? That would result in all sorts birdies and products?? |
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Thanks Joel - that's one of the pages open at least twice in this stack of Chrome tabs I'm trying to wrap my head around. ?;)
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I don't have an o-scope yet so my 'noisier' comment was also a subjective audio noise floor. My freq gen's the Adafruit board, FWIW. I was surprised to see a square wave when Ashhar probed Q7 - 2 minutes 25 in on the troubleshooting vid. ?I think that's what got me wondering about the 3 pole filter - "some" cleaning without making the output a sine wave. ?But...I'm completely new to this "RF stuff" (a technical term...) so this might just be a laugh line for the others here. ;) I'm loving the radio and the project and already want more boards to use on other bands. Andy ??
? ---In BITX20@..., <caulktel@...> wrote : Andy, Mine was noisier too until I put a LPF inline with the Si5351. I got the idea here. Scroll down the page a ways to find the filter table. Joel KB6QVI On Jan 4, 2017, at 5:01 PM, ahecker@... [BITX20] <BITX20@...> wrote:
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With a diode ring mixer, those diodes are pretty much either conducting or not conducting. ?So the action of the mixer is a square wave, even if you feed it a nice clean sine wave.
Given the balanced nature of the mixer, the narrow band crystal filter, and the band pass filter on the other side out toward the antenna, all those birdies should never come up. ?And these guys are talking broadband noise, not birdies.? Not that I know anything for sure here. ? Jerry ---In BITX20@..., <baruchatta@...> wrote : I thought that a pure sine wave was best. Isn't a square wave a mix of waves?? That would result in all sorts birdies and products?? |
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Joel At the time I built Pete's LBS, our club group build bought them. The library is bad. I'm using it to test the Bitx40 and intend to get another SI5351. Jason's sounds like a good choice. 73 Ken On Wed, Jan 4, 2017 at 10:44 PM, Joel Caulkins caulktel@... [BITX20] <BITX20@...> wrote:
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Baruch Atta
I thought that a pure sine wave was best. Isn't a square wave a mix of waves?? That would result in all sorts birdies and products? On Jan 4, 2017 10:28 PM, "Ken Chase chase8043@... [BITX20]" <BITX20@...> wrote:
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¿ªÔÆÌåÓýKen, I have used the Adafruit boards before too, but the Etherkit boards use a TXCO crystal for better temperature stability and the are only a couple of bucks more, plus I like to support Jason, NT7S because he's written most of the Libraries for the Si5351 and pushed the development for Ham radio use and built the first SSB transceiver using one. I was one of his first on the air contacts. Joel KB6QVI On Jan 4, 2017, at 7:28 PM, Ken Chase chase8043@... [BITX20] <BITX20@...> wrote:
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And I am using the Adafruit SI5351. 73 Ken On Wed, Jan 4, 2017 at 10:26 PM, Joel Caulkins caulktel@... [BITX20] <BITX20@...> wrote:
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Joel Caulkins/N6ALT
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