RX-888 UHF frequency stability

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sm6fhz
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RX-888 UHF frequency stability

#1

Unread post by sm6fhz »

Hi all.

I did some frequency drift measurements on my RX-888 purchased from "shenglongsi" eBay shop August 22nd and received on September 29th.

I waited for the RX-888 to stabilize in case temperature (+43C at +23C Room Ambient).
I then calibrated the RX-888 to a known frequency source, in this case the OZ7IGY 432 MHz beacon (controlled by a GPS locked reference).
Then I turned off the RX-888 and waited for it to cool down to Room Ambient.
After the RX-888 had stabilized at Room Ambient it was started again and the frequency of the OZ7IGY beacon was checked. It had moved from 432 471.000 kHz (as calibrated to) to 432 472.460 kHz. A movement of +1.46 kHz that is equal to +3.376 ppm.

After about one hour the RX-888 case temperature was up to +43C again and the frequency drift had stabilized. However the frequency was about 30 Hz lower than calibrated to. Maybe the crystal in the internal RX-888 is still ageing.

Another observation is that there is a clearly audible jitter on the signal at 432 MHz. This may origin from a phase locked loop with incorrect parameters in the loop filter. This jitter can clearly be seen on the waterfall display as well.

73 / Ingolf, SM6FHZ

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sm6fhz
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Re: RX-888 UHF frequency stability

#2

Unread post by sm6fhz »

Hi all.

I have now looked more at the frequency stability of my RX-888 at UHF (432 and 1296 MHz) and compared with two other SDR's covering these frequency bands.
LIME SDR USB and ADALM Pluto (clone purchased from "shenglongsi" eBay shop August 22nd and received on September 28th.
https://www.ebay.com/itm/ADI-open-sourc ... fresh=true

The ADALM Pluto clone show large frequency instabilities that probably indicates a faulty unit. Has anyone else any experience of these Pluto clones (presumably designed by Justin Peng, fei666888, https://twitter.com/fei666888, but not sold by him)? I will claim and return this faulty unit to the vendor "shenglongsi".

I used SDRC to display the signal used as stimuli where I used both a On The Air signal as well as a HP8665B signal generator with the low phase noise option and locked to a high quality ovenized 10 MHz crystal oscillator. This 10 MHz reference is calibrated to a Rubidium reference. You can in fact clearly see the difference between the different SDR's using this method.

As I mentioned above, the Pluto clone show extreme frequency instabilities (are temperature related and appears about 30 to 60 seconds after turning the unit on) as can be seen in the attached screenshots (HP8665B_432_407_MHz_Pluto and HP8665B_1296_407_MHz_Pluto). In the screenshot you can see some thermal drift after turn-on and then after about 30 seconds it just bursts into a wild jitter. I do not know the reason for sure, but it looks like a bad reference oscillator.

The LIME SDR USB is extremely frequency stable both on 432 and 1296 MHz. On 432 MHz the frequency drift from turn-on until it has stabilized is in the order of a few Hz. The frequency jitter is very low and it exhibits a very clean note when listened to. Very pleasant. See attached screenshot HP8665B_1296_407_MHz_Lime_USB.

The RX-888 shows, as reported in the first posting, an initial drift after turn-on in room ambient to thermal equilibrium of about 3.4 ppm. On top of that is has an frequency jitter that can clearly be seen on the screenshots. It does not have a clean note when listened to via SDRC. Irritating already on 432 MHz and even worse on 1296 MHz. I can not recommend the RX-888 for any serious UHF HAM usage. It would work with FM but is not acceptable for CW or SSB. I have not noticed any disadvantages at VLF, LF, MF nor HF frequencies. The reason for the thermal frequency drift as well as the frequency jitter is likely to be the reference oscillator. The jitter could also origin from a phase locked loop that does not work as it should.

73 / Ingolf, SM6FHZ
Attachments
HP8665B_1296_407_MHz_RX888.PNG
HP8665B_1296_407_MHz_Pluto.PNG
HP8665B_1296_407_MHz_Lime_USB.PNG
HP8665B_432_407_MHz_RX888.PNG
HP8665B_432_407_MHz_Pluto.PNG

Max
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Re: RX-888 UHF frequency stability

#3

Unread post by Max »

Many thanks Ingolf. Useful. Now I am put off purchase for a while!

73

Max

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sm6fhz
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Re: RX-888 UHF frequency stability

#4

Unread post by sm6fhz »

Hi all.
As a complement to the screenshots I made two audio recordings of a local 432 MHz beacon using RX-888 and LIME SDR USB feeding SDRC.
The LIME SDR USB recording is labeled 13-Oct-2020 21h22m46s, 432.435.885 MHz, CW-L
The RX-888 recording is labeled 13-Oct-2020 21h27m43s, 432.436.161 MHz, CW-L
Both are mp3-files each about 40 seconds long.
I hope this illustrates some of the difference in the note cleanness between the two.
73 / Ingolf, SM6FHZ
Attachments
13-Oct-2020 21h27m43s, 432.436.161 MHz, CW-L.mp3
(251.93 KiB) Downloaded 38 times
13-Oct-2020 21h22m46s, 432.435.885 MHz, CW-L.mp3
(305.09 KiB) Downloaded 34 times

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NO6B
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Re: RX-888 UHF frequency stability

#5

Unread post by NO6B »

The stability measurements of my RX-888 parallel yours: ~3 ppm drift, & I see what appears to be the same amount of jitter (I measured at a lower frequency: 483.9375 MHz). As my interests are almost exclusively NBFM the jitter doesn't bother me at all, but still I wouldn't call it useless for weak signal work by any means just because of that. The main problem I'm having at UHF is lack of front-end control in SDRC. With only one gain slider to work with I have to back off the gain to avoid overload to the point I lose 3 to 4 dB of S/N. OTOH using the RX-888 ExtIO DLL with HDSDR I can separately adjust the front end gain, VGA & IF to a point where it doesn't fall apart when I switch from the signal generator to the antenna. I do have a PGA-103 preamp with 20 MHz wide SAW filter ahead of the preamp; with that combo I get 0.21 uV for 12 dB SINAD. Not great but completely usable.
Attachments
RX-888 UHF freq. stability waterfall.jpg

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sm6fhz
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Re: RX-888 UHF frequency stability

#6

Unread post by sm6fhz »

Dear all.

I have to make an correction to the statements and screenshots for the ADALM Pluto clone purchased from "shenglongsi".

I found that by powering BOTH the USB-micro connectors (labeled "DATA" and "5V-IN") the wild frequency jitter, shown in the screenshots above, goes away. Left is a more moderate frequency jitter comparable to what is seen with the RX-888 at 432 and 1296 MHz (frequencies where both SDR's have coverage). The SDR starts with normal USB-power to the "DATA" USB-port but clearly do not give full/expected performance that way. Good to know.

The thermal frequency drift seems to be much better than in the RX-888 but has yet to be measured accurately. It is NOT as stable as the LIME SDR but much better than the RX-888.
Hopefully I will be able to submit some screenshots and numbers in due time.

I apologize for my incorrect statements and screenshots about the ADALM Pluto clone. S..t happens :-) but I am much happier now with a better functioning Pluto. Looks like it works great with the Satsagen S/W as well.

73 / Ingolf, SM6FHZ

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sm6fhz
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Re: RX-888 UHF frequency stability

#7

Unread post by sm6fhz »

Bob.

I guess that we all have different reference levels. For UHF weak signal work I would find the RX-888 frequency jitter quite annoying to listen to.
I agree that for FM it does not matter at all.

It is, as far I am concerned, not as good as can be done with even simple and non expensive circuits. If you compare with e.g. the LIME SDR.
But, you can not it all :-)

For the lower VHF-bands and below it is absolutely fine for weak signal work.

73 / Ingolf, SM6FHZ

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sm6fhz
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Re: RX-888 UHF frequency stability

#8

Unread post by sm6fhz »

Hi all.

I did a quick test this morning looking at the frequency drift of my ADALM Pluto clone (described above).

From turn-on until thermal equilibrium occurred, it drifted downwards about 30 Hz on 432 MHz that corresponds to ~0.07 ppm. The unit's drift stabilized in <20 minutes. The first minute after voltage was applied to the unit was not recorded as it was the time it took for the Laptop to boot and for me to start SDRC. What happens during this minute is maybe of lower interest as you are normally not able to listen anyway due to the start-up procedure ;-)

However, I think the crystal in the reference oscillator in my unit is still in it's first ageing period. It is drifting slowly upwards in frequency and had moved ~40 Hz on 432 MHz (=0.093 ppm) during the full day. This is probably a fully normal effect and will hopefully decrease quite soon.

I think these results are quite acceptable. If using it on 2400 MHz as a Tx for the EsHail up-link it would mean a turn-on frequency drift of 167 Hz. After that the drift is quite low.
I plan to do some tests to see any frequency drift triggered by Tx-ing with the ADALM Pluto. Hopefully I will be able to have some results soon.

73 / Ingolf, SM6FHZ

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