Topics

Discovery Input match

Wes Stewart
 

Has anyone measured the input match of the various filter bands?

Wes  N7WS

prog
 

On Wed, Oct 9, 2019 at 10:29 PM, Wes Stewart wrote:
Has anyone measured the input match of the various filter bands?

Wes  N7WS
This discussion could be of interest: https://twitter.com/lambdaprog/status/1183359218323283973

Ron Liekens
 

If interrested there is another nice piece of sftware that has a lot of ponential.
https://zs1sci.com/blog/nanovnasaver/#Edit

There is also a new model NanoVna in the running that will measure higher in frequency and has more dynamic range.

Regards, Ron
ON2RON

prog
 

On Sun, Oct 13, 2019 at 03:24 PM, Ron Liekens wrote:
If interrested there is another nice piece of sftware that has a lot of ponential.
https://zs1sci.com/blog/nanovnasaver/#Edit

There is also a new model NanoVna in the running that will measure higher in frequency and has more dynamic range.

Regards, Ron
ON2RON
Of course. I expect the software section to develop even further.
In any case, make sure:
1) The unit is powered on and streaming
2) The unit is tuned in the pre-selector segment you are assessing

Wes Stewart
 

I asked the question because I had already done it on two units.  I used a DG8SAQ VNWA and to summarize, we agree rather closely. 

Of course, I knew to measure each band with the radio actually tuned to a frequency in that band.  There are some subtleties; AGC, input attenation, preamp state all affect the results to some extent.  I lowered the test signal amplitude as much as possible without affecting dynamic range, to minimize these factors.

I chose to call the bands, A, B, C and D respectively.  An interesting observation is that measuring the frequency range of band "D", 17 to 31 MHz, but tuning the radio to a frequency in Band C, i.e. 16 MHz, gives a much better result.

Wes  N7WS

On Sunday, October 13, 2019, 5:45:21 AM MST, prog <info@...> wrote:


On Wed, Oct 9, 2019 at 10:29 PM, Wes Stewart wrote:
Has anyone measured the input match of the various filter bands?

Wes  N7WS
This discussion could be of interest: https://twitter.com/lambdaprog/status/1183359218323283973

prog
 

On Sun, Oct 13, 2019 at 04:43 PM, Wes Stewart wrote:
I chose to call the bands, A, B, C and D respectively.  An interesting observation is that measuring the frequency range of band "D", 17 to 31 MHz, but tuning the radio to a frequency in Band C, i.e. 16 MHz, gives a much better result.
The return loss of the 10 .. 17 MHz filter is also good for 17 .. 31 MHz because it is a 3rd order elliptic HPF.
That said, the filters were mostly optimized for the best rejection and insertion loss while still being "OK" for the return loss.
The following plot should give you an idea about what has been done.
Sig gen is a R&S SMBV100A followed by an attenuator to give a (mostly) resistive source to the unit.


Wes Stewart
 



On Sunday, October 13, 2019, 8:02:17 AM MST, prog <info@...> wrote:


On Sun, Oct 13, 2019 at 04:43 PM, Wes Stewart wrote:
I chose to call the bands, A, B, C and D respectively.  An interesting observation is that measuring the frequency range of band "D", 17 to 31 MHz, but tuning the radio to a frequency in Band C, i.e. 16 MHz, gives a much better result.
The return loss of the 10 .. 17 MHz filter is also good for 17 .. 31 MHz because it is a 3rd order elliptic HPF.
That said, the filters were mostly optimized for the best rejection and insertion loss while still being "OK" for the return loss.
The following plot should give you an idea about what has been done.
Sig gen is a R&S SMBV100A followed by an attenuator to give a (mostly) resistive source to the unit.

Okay.  I obviously have no first-hand knowledge of the internal workings of these devices.  I gleaned what I do know from reading the description on the website and by empirical measurements.  The block diagram shows the input filter as a BPF, so that's what I assumed when I read, "New High Performance Pre-selector for HF with 4 filter banks (corners at ~DC, 5, 10, 17 and 31 MHz)"

I also expected better input matching when I read: "Improved input matching down to virtually DC." Clearly, I should have asked myself, "Improved compared to what?"


prog
 

The BPFs are obtained by combining HPFs and LPFs.

Wes Stewart
 

Okay, understood.  It would be nice to see some plots of this.

Wes  N7WS

On Sunday, October 13, 2019, 11:15:14 AM MST, prog <info@...> wrote:


The BPFs are obtained by combining HPFs and LPFs.
_._,_._,_


Jos Stevens
 

Hello Prog and Wes,

Wes you asked if somebody has measured the input impedance of the Airspy Discovery,  so I tryed to measure this , using my Vector Network Analyser FA-VA3.

Please find attached my SWR graphs, for what they are worth.

The graphs show that 3.5 and 28 MHz responses are a bit schifted to the high side and at  14 an 21 MHz bands the SWR response is remarkable.

Please Prog  you speak aboput 4 filters with corners at ~DC, 5, 10, 17 and 31 MHz, are my graphs as to be expected ?

I measured the measuring voltage to be a few hundred millivolts Vtop, pretty high ofcourse , maybe too ?

I'm worryng about the response at 10 to 26 MHz.

I also mesured the parallel components of the input impedance at some  fixed frequencies :

          50 kHz : 370 Ohms // 4.2 nF

       1   MHz  : 128 Ohms // 3.1 nF

     3.65 MHz : 18 Ohms // 3.2 uH8

      7.06 MHz : 29 Ohms// 915 pF

14.25 MHz : 12.4 Ohms// 295 pF

21.219 MHz : 11.3 Ohms// 7.5 Pf

21.290 MHz : 11.2 Ohms// 7.5 pF

28.500 MHz 11.8 Ohms// 310 nH

Yes I  found the Nanovna and had ordered one,  i'm waiting for it impatiently now, then I can compare the results, and measure the higher VHF responses.

Again Prog are my measurements as te be expexted, or is there something wrong with me ??

Best 73 de

Jos PA3CCE



 



Op 9-10-2019 om 21:54 schreef Wes Stewart via Groups.Io:
Has anyone measured the input match of the various filter bands?

Wes  N7WS

Virusvrij. www.avg.com

prog
 

On Mon, Oct 14, 2019 at 02:07 PM, Jos Stevens wrote:
The graphs show that 3.5 and 28 MHz responses are a bit schifted to the high side and at  14 an 21 MHz bands the SWR response is remarkable.
Regardless of the correctness of the measurements, I want to highlight again that the SWR is much less important in receive than other parameters like the actual MDS and linearity in the desired bands and the rejection in the problematic bands. A reasonable SWR value is sufficient, but good harmonic and sub-harmonic responses are much more important for optimal performance. The latest videos from Leif SM5BSZ demonstrated how this approach compares with other radios with real life signals:

https://youtu.be/yevOmGyRhbk?t=1826
https://youtu.be/uUHr9HilTrQ?t=974

Rewind the videos for the detailed testing methodology.

Jos Stevens
 

Hi Prog and Wes,

I do understand what you mean Prog, no problem.

Please forget the 10 MHz - 26 MHz graph, SDR# was not set correctly, see the attached graph's, this time in csv format if you don't mind.

Regards,

Jos


Op 14-10-2019 om 14:42 schreef prog:
On Mon, Oct 14, 2019 at 02:07 PM, Jos Stevens wrote:
The graphs show that 3.5 and 28 MHz responses are a bit schifted to the high side and at  14 an 21 MHz bands the SWR response is remarkable.
Regardless of the correctness of the measurements, I want to highlight again that the SWR is much less important in receive than other parameters like the actual MDS and linearity in the desired bands and the rejection in the problematic bands. A reasonable SWR value is sufficient, but good harmonic and sub-harmonic responses are much more important for optimal performance. The latest videos from Leif SM5BSZ demonstrated how this approach compares with other radios with real life signals:

https://youtu.be/yevOmGyRhbk?t=1826
https://youtu.be/uUHr9HilTrQ?t=974

Rewind the videos for the detailed testing methodology.

Virusvrij. www.avg.com