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Spectrum Frequency Axis display issue in R1732 - with low sample rate

Ian DXer
 

Hi Youssef & all,

Recently I purchased a Leo Bodnar Mini GPS clock. One of the reasons for the purchase was for accurate (or improved) frequency calibration on my Airspy HF+ DP & measurements of Aussie x-band stations (more on that later). A number of issues have surfaced. I'll just deal with one in this message for the moment.

Problem: Seemingly (?) incorrect x-axis (spectrum frequency) display when sampled frequency carrier wave zoomed-in with low sample rate, pertains to display of frequency in SDR# left of the LeoBodnar GPS clock frequency generated carrier. (eg 248MHz in this case)

In regard to SDR# eg R1732 using the setup below

1. Windows 10 64bit
2. SDR# R1732
3. Sample rate 16kbps
4. Frequency: 248,000,000Hz (label H & G)
5. SDR# Display Resolution 32768 / Blackman Harris 4
6. LeoBodnar Mini GPS Clock set to output: 248,000,000Hz GPS & PLL locked
7. CW demodulation
8. Although Free Tuning mode shown in image. Carrier is centred on screen & issue occurs with Centre Tuning.

Refer to attached diagram to "see" the issue.

Basically, referring to diagram & the labels I've drawn. It seams to me that frequency labels on the spectrum display; labels B to F should be displaying 247.9999 rather than 248.000
, Label A is correct. This display anomaly only occurs when using the zoom display slider control at a significant zoom level, a low sample rate makes this all the more obvious.

Comparing this with SDR Console 3.0.20, SDR Console whilst it does have a x-axis display with an extra significant digit bit eg 247.999.98 vs 247.9999(SDR#), SDR Console is displaying carrier position correctly along the frequency x-axis correctly at high zoom levels.

Can you comment &/or investigate this Youssef?  

Thanks very much.

Ian

prog
 

On Sun, Mar 15, 2020 at 02:28 AM, Ian DXer wrote:
Can you comment &/or investigate this Youssef?  
No, this is not an issue. This is just a plain UX and taste question.
Here's the problem: The space reserved for the marked is finite, and the number of digits can only get bigger and bigger.
Someone will want another digit when zooming to the max. Then someone else will increase the decimation and a lower sample rate then asks for another couple of digits, and so on. At some point you will be rounding the displayed labels.
One of the reasons the IF plugin now displays the relative distance to the VFO and not the absolute frequency is exactly this problem.

PS: I see your unit is well calibrated!

Ian DXer
 

On Sun, Mar 15, 2020 at 12:40 PM, prog wrote:
One of the reasons the IF plugin now displays the relative distance to the VFO and not the absolute frequency is exactly this problem.
Thanks for the prompt reply & info. Yes I'd been finding the IF plugin a more useful reference during the calibration procedure.

For the benefit of others when using the Airspy HF+ DP & preforming a frequency calibration from a cold start I found I needed to give the
HF+ DP a good 2 hours warm up from an ambient air temperature of around 22-23C. The frequency drift down from 258MHz was about 6Hz before stabilizing.
Interestingly whenever I switch off SDR# & exit SDR# & start it up again immediately, there's a typical 3Hz drift down over a 3-5minute period before settling on the correctly calibrated frequency display. 

Another observation when performing a frequency calibration with SDR# is that I only get a movement of carrier frequency on display every 3-4 ppb change in the CLK (PPB) figure.
I'm guessing that's also as per design & probably really pushing beyond design capabilities of hardware/software trying to improve.

>PS: I see your unit is well calibrated!
Seams to be or should be with the effort I've put in @ least at that frequency.
That said there is a 2Hz difference between what SDR# says compared to SDR Console, can always easily compensate for that with Simon's easy 'software' calibration tool in SDR Console.
Not sure however why my Airspy seams to still be on the higher end of 0.7Hz to 1.8Hz above local remote Kiwi SDR(GPS) units for Aussie x-band stations. 
I use the carrier offsets for keeping a tab(eye/ear) on Aussie xbanders for WRTH updates/editing.
I'll be comparing frequency accuracy on other frequencies later as I'm noting some variations & not sure why yet.

Cheers.

Ian

Patrick
 

Hi Ian,

My HF+ Discovery also reads +1.1 Hz on the MW band (though perfectly calibrated on HF against various Time Stations).
Another MW Offset DX chaser ;-)

Patrick


Le dim. 15 mars 2020 à 04:20, Ian DXer <baxterinoz@...> a écrit :
On Sun, Mar 15, 2020 at 12:40 PM, prog wrote:
One of the reasons the IF plugin now displays the relative distance to the VFO and not the absolute frequency is exactly this problem.
Thanks for the prompt reply & info. Yes I'd been finding the IF plugin a more useful reference during the calibration procedure.

For the benefit of others when using the Airspy HF+ DP & preforming a frequency calibration from a cold start I found I needed to give the
HF+ DP a good 2 hours warm up from an ambient air temperature of around 22-23C. The frequency drift down from 258MHz was about 6Hz before stabilizing.
Interestingly whenever I switch off SDR# & exit SDR# & start it up again immediately, there's a typical 3Hz drift down over a 3-5minute period before settling on the correctly calibrated frequency display. 

Another observation when performing a frequency calibration with SDR# is that I only get a movement of carrier frequency on display every 3-4 ppb change in the CLK (PPB) figure.
I'm guessing that's also as per design & probably really pushing beyond design capabilities of hardware/software trying to improve.

>PS: I see your unit is well calibrated!
Seams to be or should be with the effort I've put in @ least at that frequency.
That said there is a 2Hz difference between what SDR# says compared to SDR Console, can always easily compensate for that with Simon's easy 'software' calibration tool in SDR Console.
Not sure however why my Airspy seams to still be on the higher end of 0.7Hz to 1.8Hz above local remote Kiwi SDR(GPS) units for Aussie x-band stations. 
I use the carrier offsets for keeping a tab(eye/ear) on Aussie xbanders for WRTH updates/editing.
I'll be comparing frequency accuracy on other frequencies later as I'm noting some variations & not sure why yet.

Cheers.

Ian

prog
 

DO NOT calibrate using ambient RF signals. Use hfplus_ppb.exe and a real calibrated signal generator. We have been discussing this for years. 

Patrick
 

Then why does my unit always read +1Hz when comparing - in real time - the same MW carriers - with people who run properly calibrated SDR (Perseus, G31, G33, etc) using a signal generator ?  


Le dim. 15 mars 2020 à 11:05, prog <info@...> a écrit :
DO NOT calibrate using ambient RF signals. Use hfplus_ppb.exe and a real calibrated signal generator. We have been discussing this for years. 

prog
 

On Sun, Mar 15, 2020 at 11:33 AM, Patrick wrote:
Then why does my unit always read +1Hz when comparing - in real time - the same MW carriers - with people who run properly calibrated SDR (Perseus, G31, G33, etc) using a signal generator ?  
Poor understanding of receiver architechtures.
Use Zero IF and tune near the center. 

Ian DXer
 

On Sun, Mar 15, 2020 at 09:05 PM, prog wrote:
Use hfplus_ppb.exe and a real calibrated signal generator. We have been discussing this for years.
I couldn't recall mention & usage of hfplus_ppb.exe & only really find it's usage mentioned once in msg#30767 when searching the messages.
I see the file in firmware package. Any videos showing usage for interest sake?
I used debug mode 1 to change clk as per earlier advisement if needed. Some of the early units weren't calibrated as well
as they should have been I thought I recall some saying in past & offsets lost during firmware upgrades etc.
A real calibrated signal generator would be nice to have, but all precision test equipment needs regular & probable expensive calibration too...
Anyway...I understand you don't want others getting themselves in trouble, especially when not needed with recent units etc..

>Another MW Offset DX chaser ;-)
Hi Patrick,
Not really. I find studying these local MW xband offsets exhausting :-)
Just a useful tool with Spectrum Lab etc for keeping a tab on the local band re changes here for others to benefit from.
Thanks for your interesting comments.
Cheers

Ian




prog
 

When not using the maximum sample rate, the receiver switches to Low-IF mode. This means the actual LO you are calibrating is not at the center of the spectrum, but rather at the upper edge. That's definitely not going to help with the calibration.
The solution is to use the max sample rate, tune near the center and set a very high FFT resolution. Also, it is recommended to calibrate at a higher frequency in the high VHF or the L-Band (yes, the receivers also ~work between 1200 and 1700 MHz).

jdow
 

Some people never learn, do they?

That said, you can get remarkable results against WWV on HF if you do it correctly and spend months doing so. I had a 5 MHz oven oscillator on a home built UPS back in the 70s. I built a divide by 5 million digital divider that gave a short tick once a second. I primed the pump by getting a GOOD audible zero beat on 15 MHz. Yeah, it was bounced off the moving ionosphere so iffy at best. Ya gotta start somewhere.

Then I played with a button I had setup to alter the division ratio on my divider until it's short tick lined up with WWV. I picked 5 MHz for "best" results. I checked at about the same effective time of day each day for the several years I maintained the standard. The first few days I had to the oscillator tweak mostly blind. The two pips were too far apart to be gentle. Eventually I found a setting that kept them on the scope together. That took months. The time accuracy was about 5 ms in 86400 seconds, about 60 ppb. Then came the fight between the ionosphere and real long term stability and accuracy. A couple months later had me at the point that I was remaining within about 1/4 cycle of the leading cycle 1kHz tone pulse that makes up the WWV tick. It stayed that way with tiny, really tiny, adjustments for a couple years. Then we moved. Ah well. It was fun while it lasted.

The point is you CAN do it; but, darned few people have the patience, know-how, and will to do it. Instead they "zero beat to WWV" at sunset (fastest change in delay between Ft. Collins and anywhere except a couple mile radius of Ft. Collins) and think they are ppb accurate. The math ain't there to begin with. The ionosphere ain't there. And the long observation time isn't there.

{^_^}

On 20200315 03:04:58, prog wrote:
DO NOT calibrate using ambient RF signals. Use hfplus_ppb.exe and a real calibrated signal generator. We have been discussing this for years.

jdow
 

Read my immediately prior message to this one. It is NOT "calibrated" unless you go through the steps. Depending on time of day and whether or not the other radios are colocated with yours and the readings are simumtaneous 1Hz is a SMALL error on HF.

Then ask the right question, what is the step size of the frequency synthesizer in the HF+ models AND the other "calibrated" SDRs? If you want extreme accuracy get to know your instruments intimately so you know their sources of error.

{^_^}

On 20200315 03:30:18, Patrick wrote:
Then why does my unit always read +1Hz when comparing - in real time - the same MW carriers - with people who run properly calibrated SDR (Perseus, G31, G33, etc) using a signal generator ?
Le dim. 15 mars 2020 à 11:05, prog <@prog <mailto:@prog>> a écrit :
DO NOT calibrate using ambient RF signals. Use hfplus_ppb.exe and a real
calibrated signal generator. We have been discussing this for years.

Ian DXer
 

On Mon, Mar 16, 2020 at 03:16 AM, prog wrote:
When not using the maximum sample rate, the receiver switches to Low-IF mode. This means the actual LO you are calibrating is not at the center of the spectrum, but rather at the upper edge. That's definitely not going to help with the calibration.
The solution is to use the max sample rate, tune near the center and set a very high FFT resolution. 
Thanks Prog. Very much appreciate this information.
I won't be able to achieve better results on MW X-band at present, but if I wish to attempt better freq accuracy in VHF band/channel(s) in future I'll remember your notes.
In fact I've now entered them in my own calibration doc

Prog> Also, it is recommended to calibrate at a higher frequency in the high VHF
Yes noted earlier.

Prog> or the L-Band (yes, the receivers also ~work between 1200 and 1700 MHz).
Thanks for the reminder re availability of this band. Must have a listen/play up there later :-)

jdow> what is the step size of the frequency synthesizer in the HF+ models AND the other "calibrated" SDRs? If you want extreme accuracy get to know your instruments intimately so you know their sources of error.

Absolutely! Couldn't agree more :-)  All those factors were thought of.
Back in my college years at the school of Electronics Engineering one of the expectations of our teachers were for students to account for all possible sources of error & their contribution/influence on the results in our concluding
remarks in our lab reports. Mind you having a 100kW TV transmitter tower right next door to the classrooms & student lab equipment (especially when 'some' test equipment leads were unshielded) really didn't help with some of 
results. I look back & smile ;-)





jdow
 

On 20200315 17:54:36, Ian DXer wrote:

Absolutely! Couldn't agree more :-)  All those factors were thought of.
Back in my college years at the school of Electronics Engineering one of the expectations of our teachers were for students to account for all possible sources of error & their contribution/influence on the results in our concluding
remarks in our lab reports. Mind you having a 100kW TV transmitter tower right next door to the classrooms & student lab equipment (/especially when 'some' test equipment leads were unshielded/) really didn't help with some of
results. I look back & smile ;-)
Ah memories. I talked my counselor into letting me take EE201 a semester early, the very beginning of the EE specific college courses. It's lab was called "The Toaster Lab" by the initiates. We very literally used toaster elements as resistors in the lab. We were divided into three person teams. I don't know if it was exactly accidental the freshman in a sophomore class was in a team with "the kid with professional experience." The third student was the expected level of student for the lab. Experienced and I would whirl through the measurements required with notes and all. Then we spent the rest of the hour nattering with the instructor. I'd been a ham for several years by that time and knew which end got hot and how to calculate how hot. Experienced was working part time at a TV station in Detroit at the time. He participated in their installation of color transmitters. The instructor had a similar level of experience. War stories were properly exchanged. And our third reworked the lab on his own and says he learned FAR more that way, doing it watching us then doing it himself until he got the same results really helped. College was fun. I was studying what I loved.

The kW next door and I are old friends from a couple incidents including the current setting. I have to keep on Loren to twist his test leads together to minimize the loop antenna size when he's working in the lab and I come in to kibbitz. (Yeah, we were made for each other and are joined at the hip.)

Incidentally, for those who have read this far twisted pairs work so well chiefly because they reduce the size of the loop antenna they form. (Then you get into the second and third order effects such as the different dielectric constants of white with red stripe and red with white stripe. (No fooling, you CAN see and measure that at audio frequencies!)

{^_-}