|
Any thoughts on the 'proper' selection of a low-pass filter for the output of an Si5351? ?Is it sane to think about a 60 meter low pass for the 4.7-5 MHz, or would most anything work to clean the signal as long as it passes 4.7-5?
Thanks! Andy
|
Isn't it better to drive a diode-ring mixer with a square-wave local oscillator? You're wanting to switch the diode pairs hard on, then hard off.
Steve G3TXQ
|
THIS WAS MY QUESTION ALSO. Very curious what people have to say. :)
On Wed, Jan 4, 2017 at 2:49 PM, steve@... [BITX20] <BITX20@...> wrote:
?
Isn't it better to drive a diode-ring mixer with a square-wave local oscillator? You're wanting to switch the diode pairs hard on, then hard off.
Steve G3TXQ
-- Mark Baldridge 608.561.3853
|
The diode-ring mixer operates by switching the RF signal straight through to the IF port on one half-cycle of the local oscillator, then switching an inverted version of the RF signal through to the IF port on the next half-cycle. It doesn't depend on the non-linearity of the diodes for its operation - it treats them as switches. Best inter-modulation performance comes from keeping the diodes hard on or hard off, minimizing the time for which they are in an intermediate state.
Steve G3TXQ
|
toggle quoted message
Show quoted text
On Wed, Jan 4, 2017 at 4:55 PM, steve@... [BITX20] <BITX20@...> wrote:
?
The diode-ring mixer operates by switching the RF signal straight through to the IF port on one half-cycle of the local oscillator, then switching an inverted version of the RF signal through to the IF port on the next half-cycle. It doesn't depend on the non-linearity of the diodes for its operation - it treats them as switches. Best inter-modulation performance comes from keeping the diodes hard on or hard off, minimizing the time for which they are in an intermediate state.
Steve G3TXQ
-- Mark Baldridge 608.561.3853
|
I noticed that my receiver got noisier when I disconnected the built-in VFO and added the Si5351. ?I get that a filter re-shapes the waveform, but I'm wondering about whether it's useful to clean the output signal a bit before injecting it into the radio.
I guess this is where the 'experiment' and 'learning' parts of the game happen. :)
Andy
|
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
toggle quoted message
Show quoted text
On Jan 4, 2017, at 5:01 PM, ahecker@... [BITX20] < BITX20@...> wrote:
?
I noticed that my receiver got noisier when I disconnected the built-in VFO and added the Si5351. ?I get that a filter re-shapes the waveform, but I'm wondering about whether it's useful to clean the output signal a bit before injecting it into the radio.
I guess this is where the 'experiment' and 'learning' parts of the game happen. :)
Andy
|
Joel when you say noisier, do mean sounds noisier or scope trace is noisier? I built the top filter and I didn't hear any difference. On the scope, the signal cleaned up good, but I could see some phase noise.
73
Ken VA3ABN
On Wed, Jan 4, 2017 at 8:26 PM, Joel Caulkins caulktel@... [BITX20] <BITX20@...> 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
?
I noticed that my receiver got noisier when I disconnected the built-in VFO and added the Si5351.? I get that a filter re-shapes the waveform, but I'm wondering about whether it's useful to clean the output signal a bit before injecting it into the radio.
I guess this is where the 'experiment' and 'learning' parts of the game happen. :)
Andy
|
Ken,
I also noticed a clean output after the LPF with Phase noise as well, but I also noticed a lower background noise floor in receive, in fact I'm very happy with it now. If it wasn't for the hiss of the LM386, it would be a most quiet receiver. Joel? KB6QVI
toggle quoted message
Show quoted text
On Jan 4, 2017, at 6:07 PM, Ken Chase chase8043@... [BITX20] < BITX20@...> wrote:
?
Joel when you say noisier, do mean sounds noisier or scope trace is noisier? I built the top filter and I didn't hear any difference. On the scope, the signal cleaned up good, but I could see some phase noise.
73
Ken VA3ABN
|
Thanks Joel. Since I built it, I may as well use it.
73 Ken
On Wed, Jan 4, 2017 at 9:52 PM, Joel Caulkins caulktel@... [BITX20] <BITX20@...> wrote:
?
Ken,
I also noticed a clean output after the LPF with Phase noise as well, but I also noticed a lower background noise floor in receive, in fact I'm very happy with it now. If it wasn't for the hiss of the LM386, it would be a most quiet receiver. Joel? KB6QVI
?
Joel when you say noisier, do mean sounds noisier or scope trace is noisier? I built the top filter and I didn't hear any difference. On the scope, the signal cleaned up good, but I could see some phase noise.
73
Ken VA3ABN
|
I'm no RF guru, but had thought?a diode ring mixer would be fine with a square wave LO. Would be interesting to hack the analog VFO to give a square wave at the same amplitude as the Si5351, see if that has similar rx noise. ?Might be other things going on here, like uC hash or a poor board layout. Curious how the softrock guys found it necessary to have galvanic isolation for their Si570.
I assume nobody in the field yet has a Raduino equipped Bitx40 to experiment with. What Si5351 board are you using?
Jerry, KE7ER
On Wed, Jan 4, 2017 at 8:26 PM, Joel Caulkins caulktel@...? wrote:? ?Mine was noisier too until I put a LPF inline with the Si5351.?
|
Jerry,
Im using the Etherkit Si5351 breakout board.? Joel KB6QVI
toggle quoted message
Show quoted text
On Jan 4, 2017, at 7:08 PM, jgaffke@... [BITX20] < BITX20@...> wrote:
?
I'm no RF guru, but had thought?a diode ring mixer would be fine with a square wave LO. Would be interesting to hack the analog VFO to give a square wave at the same amplitude as the Si5351, see if that has similar rx noise. ?Might be other things going on here, like uC hash or a poor board layout. Curious how the softrock guys found it necessary to have galvanic isolation for their Si570.
I assume nobody in the field yet has a Raduino equipped Bitx40 to experiment with. What Si5351 board are you using?
Jerry, KE7ER
On Wed, Jan 4, 2017 at 8:26 PM, Joel Caulkins caulktel@...? wrote:? ?Mine was noisier too until I put a LPF inline with the Si5351.?
|
And I am using the Adafruit SI5351.
73 Ken
On Wed, Jan 4, 2017 at 10:26 PM, Joel Caulkins caulktel@... [BITX20] <BITX20@...> wrote:
?
Jerry,
Im using the Etherkit Si5351 breakout board.? Joel KB6QVI
?
I'm no RF guru, but had thought?a diode ring mixer would be fine with a square wave LO. Would be interesting to hack the analog VFO to give a square wave at the same amplitude as the Si5351, see if that has similar rx noise.? Might be other things going on here, like uC hash or a poor board layout. Curious how the softrock guys found it necessary to have galvanic isolation for their Si570.
I assume nobody in the field yet has a Raduino equipped Bitx40 to experiment with. What Si5351 board are you using?
Jerry, KE7ER
On Wed, Jan 4, 2017 at 8:26 PM, Joel Caulkins caulktel@...? wrote:? ?Mine was noisier too until I put a LPF inline with the Si5351.?
|
Ken, 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
toggle quoted message
Show quoted text
On Jan 4, 2017, at 7:28 PM, Ken Chase chase8043@... [BITX20] < BITX20@...> wrote:
?
And I am using the Adafruit SI5351.
73 Ken
|
|
|
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:
?
Ken, 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
?
And I am using the Adafruit SI5351.
73 Ken
|
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??
|
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. ?;)
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
?? | | | | | | A short video showing how to troubleshoot the receiver of the BITX40 with a piece of wire | | | | Preview by Yahoo | | | ?
---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:
?I noticed that my receiver got noisier when I disconnected the built-in VFO and added the Si5351. ?I get that a filter re-shapes the waveform, but I'm wondering about whether it's useful to clean the output signal a bit before injecting it into the radio.
I guess this is where the 'experiment' and 'learning' parts of the game happen. :)
Andy
?
|
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??
|
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
|
Joel Caulkins/N6ALT