RFS 1000 Off Air Frequency Standard - Page 1
Forum section: electronics / projects

author: Mike99
author_rank: Frequent Contributor
timestamp: January 27, 2024, 03:20:48 pm
content: Does anyone have any info on this device? User manual or schematic?I picked it up at a radio rally and it works (until 198kHz is switched off ...) but I'd like to understand what some of the switches are for.Mike

author: G0HZU
author_rank: Super Contributor
timestamp: January 27, 2024, 03:45:32 pm
content: I've not seen that model before, but it looks like the left most switch is just the on/off switch.The other switches might be as follows:SYNC I'm not sure about this one. It might just be a control that lets you keep the output running but without trying to sync to 198kHz. So if SYNC is off then it runs open loop. If SYNC is on then it tries to sync to 198kHz.BAT battery status. This might allow the meter to show the battery voltage? (if it can run with internal batteries?)PHASE I'm not sure about this one either. This might let the meter show the error/control correction voltage for the VCO when locked to 198kHz. If it is locked it will probably sit somewhere near the middle of the meter range if the VCO is trimmed correctly. The front panel has a VCO trim control that is probably tweaked (during a routine service) to make sure this is the case?

author: Mike99
author_rank: Frequent Contributor
timestamp: January 27, 2024, 08:00:03 pm
content: Quote from: G0HZU on January 27, 2024, 03:45:32 pmThanks for your suggestions. When the unit is turned on the LED is solid until it locks, then it flashes (and beeps annoyingly!). The frequency error is 0.5Hz on the 1MHz setting and the meter reads about half scale.The SYNC button is spring loaded. The unit continues to flash/beep when it's depressed so I'm not sure what it does.The unit runs off a 9V battery and the BAT button gives a relative indication of its voltage.When the PHASE button is pressed the LED and beeping stop and the meter swings up and down. The trim control doesn't turn and I don't want to force it.Mike

author: G0HZU
author_rank: Super Contributor
timestamp: January 27, 2024, 10:04:57 pm
content: OK Thanks.It may be the case that (during a routine service), the operator presses and holds the spring loaded SYNC button to put the unit in open loop (and pressing the SYNC button maybe also forces the internal tune voltage to mid range to aid calibration).I think this would allow the VCO to be trimmed to get the unit exactly on 1MHz in open loop (no SYNC operation)Then they release the SYNC button and hopefully the unit will acquire lock within a few minutes. If all is well, the PHASE display would settle mid scale once locked.Maybe take the lid off and see if pressing the SYNC button forces the tuning voltage to mid range. This would be the calibration position to make sure that in open loop the VCO is running at 1MHz at the centre of the tuning range. If yours can't do this then maybe it is faulty or out of adjustment.But I can only guess. Maybe wait to see if someone has already used one of these.

author: Mike99
author_rank: Frequent Contributor
timestamp: January 27, 2024, 10:21:03 pm
content: OK thanks again, I will open it up when I get a moment. I don't have a serious use for it as I already have a GPSDO but I do like collecting vintage equipment.Mike

author: 807
author_rank: Frequent Contributor
timestamp: January 27, 2024, 11:25:01 pm
content: By coincidence, I had a play with my RFS 1000 recently. I found it while rummaging through some old test gear I bought on Ebay decades ago. I thought I was the only person on the planet to have one of these, considering there is almost no mention of them on the internet.You say that yours is 0.5Hz out, but even though I get a 1 second beep, and my signal strength is 8, the 1MHz output is very jittery. I can't get a stable reading on mine. The frequency counter jumps about between around 999.997kHz to 1000.003kHz. It probably averages 1MHz I suppose?The phase switch sets the meter to between 4 & 5 most of the time, but it occasionally spikes to 10.Did you notice the longer beep at the start of every minute?

author: G0HZU
author_rank: Super Contributor
timestamp: January 28, 2024, 12:00:54 am
content: If these are really old, are you both certain these were designed for 198kHz? Radio 4 used to be on 200kHz so early offair standards (from early manufacturers) were designed for 200kHz. Some could be configured to support both 200kHz and 198kHz.

author: 807
author_rank: Frequent Contributor
timestamp: January 28, 2024, 12:22:04 am
content: Quote from: G0HZU on January 28, 2024, 12:00:54 amActually I've just discovered it's not 198kHz or 200kHz, it's an MSF 60kHz receiver!I dangled an aerial from my sig gen set at 198kHz over the standard thinking it would blot out the original signal, but it had no effect. Setting the sig gen to 60kHz kills it.I'll open it up in the morning & take a peek inside.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 28, 2024, 09:24:41 am
content: A few years ago I built a 60 KHz off air frequency standard by Andy Talbot, this page has the details:http://g4jnt.com/freqlock.htmThe RFS1000 is probably similar to the above. They are easy and fun to make, although 60 KHz is prone to interference from all manner of household electronics, wall wart power supplies, etc.Expect a stability/jitter of around 1 part in 10^8 with a clean MSF signal, or perhaps slightly better as the unit appears to use a TCXO. They are nowhere near as stable as a good home made GPSDO (such as a Lars Walenius one described elsewhere on the EEV Blog Forums, and also athttps://www.qsl.net/zl1bpu/PROJ/NGPSDO/New%20GPSDO.htm).SJ

author: 807
author_rank: Frequent Contributor
timestamp: January 28, 2024, 12:41:41 pm
content: @Solder_Junkie. I found the original MK1 article, but couldn't find the MK2 last night. So thanks for the links.I tried it again this morning, thinking it might be more stable during daylight hours, bit it's still just as jittery.I tried playing with the VCO trim. It looks like it should be set to give a half way reading on the phase meter. It goes out of lock if adjusted too far from this.The signal meter never seems to move. It always reads 8, until I put it in the microwave. It went out of lock & the signal meter very gradually moved down to zero. When I took it out, it very slowly went back to 8. So the signal meter seems very heavily damped.I've taken a look inside. There are 2 metal boxes with lids soldered on one corner. I'll need to find my big soldering iron to get them off. Stickers say it was tested on 14-3-92.

author: G0HZU
author_rank: Super Contributor
timestamp: January 28, 2024, 01:03:50 pm
content: Funnily enough I'm playing with some offair standards this weekend as part of a research task. The main interest is the long term accuracy of GPSDO, especially with a less than perfect (or mobile) antenna.I'm looking at the phase response using a phase meter and logging it over an hour or so. However, I've also tested a Quartzlock 2A-Y 198kHz offair standard and also my old homebrew 198kHz offair standard. I designed it myself just over 30 years ago and it is still working fine. The loop filter caps are slowly drying out so it is getting a bit undamped. But it still locks in about 100 seconds from a cold start.Here's an old youtube video showing my homebrew standard showing that it can quickly achieve about 0.01Hz or better at 10MHz output.The Quartzlock 2A-Y takes much longer to lock but it gives similar performance once locked. It also uses a huge coaxial loop antenna which is supposed to minimise phase shifting caused by the alternative ferrite loop antenna. My homebrew standard seems to run fine with a ferrite rod antenna. In my experience, the worst time to try and use the 198kHz standards is just before dusk because the groundwave and the initial appearance of night time propagation seem to conflict and cause poor performance. I'm only 20 miles from Droitwich so I get really good reception here.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 28, 2024, 01:24:23 pm
content: Quote from: G0HZU on January 28, 2024, 01:03:50 pmWhat phase meter are you using? I have a TinyPFA and a rubidium standard. The monitoring software is TimeLab.My main interest is in short term stability for both an external standard input with test equipment and as an external reference with my amateur radio equipment.While the short term stability of the rubidium is not something I can realistically measure, it is interesting to look at the jitter/short term frequency deviation of the three GPSDOs I have. I am primarily interested in accuracy/stability over a period of around 10 seconds. The JNT 60 KHz unit was donated to a friend some time ago, but from memory and “old fashioned” dual beam oscilloscope comparison with my rubidium, it was approximately 1 part in 10^8 short term.SJ

author: CaptDon
author_rank: Super Contributor
timestamp: January 28, 2024, 09:07:28 pm
content: That seems like a nice thing to have an entertainment radio station also be an accurate frequency standard. Being currently in the states it would be a nice thing if our entertainment 1 MHz radio stations were GPSDO locked at exactly 1 MHz and could be used as a reference. We do have all of the WWV stuff at 2.5, 5.0, 10.0, 15.0, and 20.0 MHz. They diddled with the 60KHz signal flipping phase every so often which made the old Spectracom stuff obsolete unless modified. As a mariner I was very sad when the Loran C system was turned off. The government had spent about 100 million upgrading the sites to solid state transmitters and modulators and then shut the system down. GPS is nice but Loran was always a good plan B and was useful to establish agreement between the GPS and Loran units. I don't think E-Loran ever got off the drawing board and into reality. With pier mounted GPS correction transmitters I can position the vessel within 1 meter / 3 feet. I think Loran C was about 50 feet when everything was 'perfect' which wasn't often.

author: G0HZU
author_rank: Super Contributor
timestamp: January 28, 2024, 11:28:31 pm
content: Quote from: Solder_Junkie on January 28, 2024, 01:24:23 pmI'm using an old HP 8405A vector voltmeter for the phase monitoring. I'm not sure how it compares to the TinyPFA in this respect. It can resolve down to about 0.01degree via the DVM recorder port at the back. It really is old test gear but it works great for the stuff I'm doing as it gives a fast and fluid output of phase error vs time.I'm not sure what happens after March 2024 when Radio 4 stops transmitting on 198kHz. It may be that the transmitter will just transmit the teleswitch data. So my offair standard will keep working.otherwise, I'm not sure how accurate the 693kHz station is from Droitwich. I can remember testing it a few years ago with a long data grab using an RTSA and the output seemed to be as close to 693kHz as I could measure. So it might be locked to the same reference as 198kHz?I like using an offair standard because the lock time is so fast. Only 90 seconds from a cold start in my case. By contrast, a GPSDO can take a long time and it really needs a good antenna to work really well.It would be interesting to see inside the RFS1000.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 29, 2024, 08:32:13 am
content: Quote from: G0HZU on January 28, 2024, 11:28:31 pmI think the problem with longwave and Radio 4 is the cost of running the transmitter for the few listeners it has, and the irreplaceable output valve(s) it uses... so come March the carrier will cease. Sadly it will also mean mariners will no longer have the shipping forecast.Depending on how far you are from Anthorn, and how strong MSF is, the other option is to convert to 60 KHz. Another option could be to use a Ublox GNSS (GPS) module to output 198 KHz and feed that into your "Radio 4" off air standard. I have a spare module, I will give it a try and see how clean (or dirty) the 198 KHz is.SJ

author: Mike99
author_rank: Frequent Contributor
timestamp: January 31, 2024, 01:50:32 pm
content: Quote from: 807 on January 27, 2024, 11:25:01 pmGreat to meet some more RFS 1000 owners! We must be a rare breed.My signal strength is 7 and with the phase switch down the meter reads 4. I do get a longer beep but if I had a serious use for this I would disable it altogether. Does my VCO trim control look OK or is it missing a knob?Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: January 31, 2024, 01:51:45 pm
content: Quote from: G0HZU on January 28, 2024, 12:00:54 amThe guy who sold it told me it was 198kHz but I'll have to check.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: January 31, 2024, 01:57:49 pm
content: Quote from: Solder_Junkie on January 28, 2024, 09:24:41 amI built the Lars GPSDO a few years ago and tested it extensively with different OCXOs using TimeLab. Then I came across this one, which I prefer. It does take a long time to lock but it's nice to see 10,000,000.000Hz on the displayhttps://www.instructables.com/GPSDO-YT-10-Mhz-Lcd-2x16-With-LED/I built it with the ublox M8T timing receiver.Mike

author: 807
author_rank: Frequent Contributor
timestamp: February 01, 2024, 11:07:58 am
content: Quote from: Mike99 on January 31, 2024, 01:50:32 pmMy VCO trim is the same. It's the shaft of a polyvaricon mounted (glued?) behind the front panel. I know that those type of trimmers sometimes stick when not used for a while. Mine moved freely though.Since buying it, I bought a few rubidiums & a couple of GPSDO standards. I'm currently using the Leo Bodnar mini GPS as my test gear standard. I don't know whether my RFS1000 is working correctly. It appears to lock OK, but like I say, the jitter is unacceptable as a proper standard.I can't find my big soldering iron to desolder the lids of the metal cans. I'll try using my normal soldering iron later today to open them up.

author: 807
author_rank: Frequent Contributor
timestamp: February 01, 2024, 11:17:02 am
content: Quote from: Mike99 on January 31, 2024, 01:51:45 pmLike I said earlier, when I flooded the unit with a 198kHz signal it had no effect, but a 60kHz signal killed it, even with the generator set to around -30dBm with a piece of wire dangling from the output socket.I also assumed it was 198kHz. I can't remember what the original Ebay listing said, as it was bought in 2008. I paid £71.99 for it. Can't remember wher it was auction or BIN.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 01, 2024, 12:04:49 pm
content: Quote from: 807 on February 01, 2024, 11:07:58 amDo you have a means of plotting one of your rubidium oscillators against the GPSDOs? I have a TinyPFA, which is based on the NanoVNA H4 and is interchangeable if you load the appropriate firmware. My Leo Bodnar is not anywhere as clean is my home made Lars Walenius GPSDO, although it is convenient as it (a) locks very quickly and (b) it can output almost any frequency I need.See attached image for a comparison at 10 MHz using the TinyPFA and TimeLab. The red trace is the Lars and the green my Leo Bodnar. The reference is a rubidium, which itself may not be the most stable reference, but it's the only rubidium I have and comparisons always produce repeatable results. Both GPSDO units were using the same roof mounted "mushroom" antenna from Leo Bodnar.SJ

author: 807
author_rank: Frequent Contributor
timestamp: February 01, 2024, 01:34:07 pm
content: Quote from: Solder_Junkie on February 01, 2024, 12:04:49 pmUnfortunately I don't have the NanoVNA H4, I have the NanoVNA-F v2, so can't convert it to a PFA. I might buy an H4 just to convert it to a PFA though. 🤔Quite a difference in those plots! My other GPSDO's are a couple of Thunderbolts.My only criteria at the moment is accuracy by measuring the frequency on a counter & how clean it looks on a spectrum analyzer.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 01, 2024, 01:57:41 pm
content: The Thunderbolts are reckoned to be really good. It’s a pity your VNA isn’t the H4 version.SJ

author: Mike99
author_rank: Frequent Contributor
timestamp: February 01, 2024, 02:50:23 pm
content: Quote from: 807 on February 01, 2024, 11:17:02 amI've just done the same test and mine is 60kHz also. I got it for £10 as I think the previous owner was expecting it to stop working soonI was wrong about the accuracy, I misread my Racal frequency counter. It jumps between +400Hz and +500Hz on 1MHz.Mike

author: 807
author_rank: Frequent Contributor
timestamp: February 01, 2024, 08:09:56 pm
content: Quote from: Solder_Junkie on February 01, 2024, 01:57:41 pmThe Leo Bodnar unit is so compact & versatile compared to the Thunderbolt though. The Thunderbolt requiring +5v, +12v & -12v supplies.I'm interested in taking some meaningful readings now, so might buy myself an H4.

author: 807
author_rank: Frequent Contributor
timestamp: February 01, 2024, 08:16:51 pm
content: Quote from: Mike99 on February 01, 2024, 02:50:23 pmI'm using a Racal Dana too. A 1998. Yeah, it's not very stable. As they're sold as frequency standards I thought they would be better than that.I've taken the lids off the cans. I'll take some photos & post them up.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 01, 2024, 08:43:42 pm
content: I also have a Racal 1998. The ovens were excellent at one time, but they are up to 36 years old. I have a manual dated 1987.The oven in mine is the 04B option, but even though it’s the highest stability one, it doesn’t have the accuracy or stability mentioned in the manual. Maybe if I left it running for several days it might improve, but having 3 GPSDOs and a rubidium, it is not worth bothering with the internal oven.SJ

author: 807
author_rank: Frequent Contributor
timestamp: February 01, 2024, 08:55:53 pm
content: My 1998 has the bog standard (black box) oscillator. I have a few OSC5A2B02 OCXO units that I intend to replace it with (along with another dozen or so projects on my list).As you say, with a better standard plugged in the back it doesn't really matter, but on the off chance that I might need to use it somewhere else, it would be nice to know it has the best built-in reference possible.

author: 807
author_rank: Frequent Contributor
timestamp: February 01, 2024, 08:57:53 pm
content: I've attached 2 photos of the RF & logic boards inside the metal cans. The 3 chips on the right side of the RF board have had their markings removed. All the chips on the logic board are still readable. The yellow & purple wires at the top right of the logic board go to the front panel VCO trim control.The 10kHz output is taken from the ribbon cable (you can see the gap in the ribbon cable where the black lead under my thumb is soldered to it). The other frequency outputs are taken from the ribbon cable that connects to the front panel.

author: Mike99
author_rank: Frequent Contributor
timestamp: February 01, 2024, 09:28:11 pm
content: Thanks for the photos 807 (BTW the last time I saw an 807 was in the PA of a No. 19 set in the late 1960s).I suppose the logic board might be useful to provide divided outputs in another more accurate frequency standard. Do you know what the input frequency is?Also will the outputs of this device drive 50 ohms?Mike

author: 807
author_rank: Frequent Contributor
timestamp: February 02, 2024, 03:03:24 pm
content: Yep. I had a number 19 set many years ago. Managed to get the receiver working, but not the TX. Since then I've built a few transmitters using 807's. I used to be a medium wave radio pirate. Still got my old vave rig in the loft that I built in the late 70's.I probed a few components on the RF board & could see the 60kHz input. I thought about taking the board out to reverse engineer it, but there are 2 blobs of solder on the edge of the board soldered to the can. I really need to find my big soldering iron before attempting to remove it.When I used my sig gen, I noticed that the peak output was at 55kHz, so I might try adjusting the green trimmer in the top left of photo.The outputs are not 50 Ohms. All the outputs measured 5v5 p-p on scope, but when fed through a 50 Ohm terminator, the p-p dropped to 550mv. Strangely, when the 10kHz output was connected to 50 Ohms, the signal went down to zero with a continuous tone on the beeper.

author: Mike99
author_rank: Frequent Contributor
timestamp: February 03, 2024, 07:03:39 pm
content: Quote from: 807 on February 02, 2024, 03:03:24 pmYes I found the same.What is the correct procedure for adjusting the vco trim control?Mike

author: 807
author_rank: Frequent Contributor
timestamp: February 04, 2024, 08:52:29 pm
content: I assume that the VCO trim should be set for half way reading on the phase meter. The unit goes out of lock if it's adusted too far from this point.Something I forgot to mention in my last post. Only the 1MHz output is a square wave. All the other outputs are pulses, with a 20% duty cycle.

author: Mike99
author_rank: Frequent Contributor
timestamp: February 06, 2024, 09:56:55 am
content: OK thanks. My phase swings between 4 and 6 on the meter with occasional larger excursions so that sounds about right. What about the Sync switch, what does that do?Agreed on the waveforms, here are mine.Mike

author: 807
author_rank: Frequent Contributor
timestamp: February 07, 2024, 12:52:34 pm
content: My phase meter swings about a bit too. Sometimes it's quite stable, other times it bangs against the end stops!The sync switch is still a bit of a mystery. It doesn't appear to have any effect on the meter or the outputs. I traced it back to the logic board & it connects the output of one of the AND gates on IC17 to the RESET A & RESET B pins of the dual BCD up conters (IC15 & 16).