Suggestions for a cheapo RF Signal Generator - Page 1
Forum section: electronics / testgear

author: stuartk
author_rank: Regular Contributor
timestamp: February 29, 2016, 01:49:06 am
content: So I came into this old Fluke PM6680 225Mhz frequency counter for cheap. It has a due for calibration in 2015 sticker on it. I'm pretty sure they sold it as the bottom of the case was dented inwards, making opening of the unit virtually impossible. It's one of those "slide out" pcb's and the back panel was stuck on the dent. This particular model requires opening it up to calibrate it.With a not so deft touch and little patience, I was able to open the damn thing up. A crowbar has straightened out that old time cast iron case. It seems to work reasonably well, within 5Hz of my 10MHz OCXO, both before and after the crowbar.So I decided to purchase the 3Ghz prescaler to 'upgrade' it. I could use the counter portion of the unit for measuring pulse events but truth be told I don't actually do any RF stuff. I really don't need a 3GHZ frequency counter. I'm just doing this for fun. Except it turns out the 3GHz prescaler requires a 1 GHz signal to calibrate it. d'oh!This led me to an investigation of cheapo signal generators; So in order of cheapness, here they are.....$57.99 TFT stand alone generatorThis one actually looks surprisingly good for the money. I know I couldn't do it for $57.http://www.aliexpress.com/item/ADF4350-DDS-RF-Signal-Generator-35M-4-4G-TFT-LCD-Development-board-STM32F103/32581813036.html?spm=2114.01020208.3.144.vW1evT&ws_ab_test=searchweb201556_1,searchweb201644_2_505_506_503_504_301_10020_502_10001_10002_10017_10010_10005_10006_10011_10003_10021_10004_10022_10009_10008_10018_10019,searchweb201560_5,searchweb1451318400_-1,searchweb1451318411_6450&btsid=224d27b7-0ff7-411c-b59a-5f9449f63027$60.78, USB generatormore Chinese goodness, but ugh the software!http://www.ebay.ca/itm/NEW-138MHz-4-4GHz-USB-SMA-Source-Simple-Spectrum-Analyzer-Signal-Generator-/181789427266?&autorefresh=true$175 ++for shipping. Analog ADF4351 Evaluation board from Analog devices.I hear the eval board software it a bit of a pain to use.http://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/eval-adf4351.html#eb-overview$195 TPI synthesizer. USBSoftware looks easier to use, and actively supported.http://www.rf-consultant.com/simple-signal-generator/$319 6Ghz USB generator from DS InstrumentsSoftware looks polished. Clean designhttps://www.dsinstruments.com/store/products/micro-rf-signal-generator-sg6000m/$599 Tracking generator from Signal Hound.Looks like a more serious instrument. It would pair well with their USB spectrum analyzerhttps://signalhound.com/products/usb-tg44a/$1040 Used Agilent E4421B 3GHz GeneratorDespite it's obvious nerd appeal, It would likely be an expensive paperweight for me.http://www.ebay.ca/itm/Agilent-E4421B-opt-1EM-/281938934527?hash=item41a4debaff:g:9g0AAOSwB4NWxW~KIf anyone lives near the Ottawa area and can spare an hour on the weekend to let me borrow the use of their RF signal generator, let me know.Thanks, Stuart

author: uncle_bob
author_rank: Unknown
timestamp: February 29, 2016, 01:58:36 am
content: HiIf it's a one time deal and then you never need it again ... rent one. A real generator will probably be what the cal procedure is looking for . A lash up with a demo board may be a real PIA. It all depends on just what your pre-scaler alignment process looks like.Bob

author: stuartk
author_rank: Regular Contributor
timestamp: February 29, 2016, 02:17:27 am
content: The cal procedure is as basic as it comes. Just twist the trimmer potentiometer on the board until the frequency counter displays 1 GHz.

author: uncle_bob
author_rank: Unknown
timestamp: February 29, 2016, 02:23:53 am
content: Quote from: stuartk on February 29, 2016, 02:17:27 amHiWell normally there is a "apply -20 dbm at 1 GHz" phrase just before the twist the pot (or what ever it is). I think I would want to check that the gizmo really made it to 3 GHz before I was done with the generator. For practical use, you sort of need to know what sort of range of levels it runs over.Bob

author: stuartk
author_rank: Regular Contributor
timestamp: February 29, 2016, 03:09:14 am
content: This is the information they give:

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 29, 2016, 03:49:09 am
content: Hi,The calibration is to adjust the trigger level. The generator needs to have an accurate output level. The frequency is not super critical. The frequency division is a digital thing and should not need adjustment.Can you post a link to the upgrade kit/board?Regards,Jay_Diddy_B

author: smbaker
author_rank: Regular Contributor
timestamp: February 29, 2016, 05:13:59 am
content: Quote from: stuartk on February 29, 2016, 01:49:06 amI own this one, and am very pleased with it. The software is well designed. I was actually using it this very evening working on a frequency counter project.Scott

author: Ivan7enych
author_rank: Regular Contributor
timestamp: February 29, 2016, 10:51:01 am
content: Quote from: stuartk on February 29, 2016, 01:49:06 amI have this and would not recommend it as a standalone unit. It's mostly an addon to spectrum analyzer, not designed to be a fulli functional standalone unit. As a generator it has minimalistic software - only frequency and power, that's it. Sweep function for this unit was written by myself based on it's API. It outputs square wave (with lots of harmonics) on frequencies > 30MHz. It doesn't have 10MHz reference input. The only good thing - it has internal attenuator with 1dB step and -10dBm ... -40dBm output power range.Quote from: stuartk on February 29, 2016, 01:49:06 amThis thing should output pure sine (very small harmonics, unlike all chinese cheap units), wide range of output power and have some modualtion options.

author: nctnico
author_rank: Super Contributor
timestamp: February 29, 2016, 02:58:34 pm
content: Quote from: Ivan7enych on February 29, 2016, 10:51:01 amThe sine wave coming from most RF generators isn't very pure. 2nd order harmonics are commonly around -30dB (IIRC -40dB for the E4400 series). AFAIK the problem is that RF amplifiers don't do low distortion very well. The advantage of a 'real' RF generator is that it will have a well controlled amplitude and various signal generation & modulation posibilities. If you are looking for just a signal a simple board will do just fine.

author: stuartk
author_rank: Regular Contributor
timestamp: February 29, 2016, 03:37:29 pm
content: Quotehttp://www.ebay.com/itm/Prescaler-for-Philips-FLUKE-PM6685-PM6680-PM6681-Frequency-Counters-/181841494847?

author: Howardlong
author_rank: Super Contributor
timestamp: February 29, 2016, 04:06:57 pm
content: I find that a reasonably well calibrated RF signal generator is the single most used piece of RF test equipment in the lab. you can test sensitivity, inject known, controlled signals and us it to calibrate your other equipment, for example if you want to use a cheap SDR as a narrow band spectrum analyser.I don't know how well calibrated these cheap one or two chip USB based RF signal generators are, I can't imagine their output level is too trustworthy across their entire coverage. For measuring sensitivity, you absolutely need a reasonably well calibrated RF signal generator or all bets are off. For measuring and comparing frequency, then assuming they have a reasonable timebase then they will be useful. if that's what you need, but if phase noise concerns you then they might not be the best.As has been covered in other threads, a radio or communications test set can be a reasonable alternative, and gives you a few extras.If all you're doing is using third party pre-baked RF modules, then the value of a signal generator is somewhat limited, it's only when you're rolling your own that they become incredibly useful.

author: LogicalDave
author_rank: Regular Contributor
timestamp: April 08, 2020, 11:45:16 am
content: >> The frequency is not super critical. The frequency division is a digital thing and should not need adjustmentThe frequency is generated from some analog reference source: hopefully at least a TCXO, but on low cost units possibly just a simple XO. A PLL multiplies the crystal frequency (and error) and digital dividers then convert the frequency to the one you want. Crystals drift in frequency over time and temperature so calibrating the frequency may be important; consider that a 50ppm crystal can be off frequency by 50kHz at 1GHz.If your signal generator isn't generating an accurate frequency or an accurate signal level, then what exactly is it good for?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 08, 2020, 12:37:54 pm
content: LogicalDave,Quote from: LogicalDave on April 08, 2020, 11:45:16 amThis was in the context of the previous message:The CAL procedure is asking for 1GHz +/- 25MHz2.5% or 25000 ppmThis is this specific case, I agree with you that you would normally want much better than this in a general purpose signal generator.Regards,Jay_Diddy_B

author: bob91343
author_rank: Super Contributor
timestamp: April 08, 2020, 05:18:18 pm
content: It strikes me as odd that you are willing to spend so much money to calibrate a device for which you don't anticipate a need. Yes I know it's for fun but still...A few years ago I bought a Chinese counter that goes to 2.4 GHz. It was very cheap and even has a TCXO reference.Much of my test equipment is only for fun. I have to remember, occasionally, to run some of it just to make sure it still works. I have always been fascinated by test gear and making precise measurements. It felt like being out of control when I would make a measurement, as I never knew if it was accurate.I even got a rubidium standard to caibrate my counter and signal generators.