Biased diode detector - without bias - Page 1
Forum section: electronics / rf-microwave

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 17, 2025, 12:29:49 am
content: Hi!I'm trying to characterise a label-less but very nice looking diode detector. Nothing is known about it.It has a solder post for bias, so I assume it's a schottky detector. How would one go about figuring out the bias voltage, and can biased diode detectors be operated without bias?The construction is really neat. From the SMA: pad attenuator, tuning screw (tiny capacitor?), diode element, larger capacitor, stripline choke, bias through an SMD device, ferrite bead around SMB pin.I haven't dared to set a multimeter to it, as I assume the diode is fragile beyond belief.I guess it's a pretty standard circuit, but it looks pretty well laid out.Does anyone recognize the design or package?

author: iMo
author_rank: Super Contributor
timestamp: August 17, 2025, 06:44:30 am
content: You may try to test it - with a small bias like 1-10uA forward current and a small input voltage, like 10-100mVpp (say at 100+MHz) you should see the difference in the output voltage (with bias vs. none bias). The small bias current usually improves a little bit the linearity of the detector at lowest signal levels..

author: G0HZU
author_rank: Super Contributor
timestamp: August 17, 2025, 11:43:14 am
content: It looks to me like it could be a tuned detector so it will probably be very narrowband, somewhere up at high VHF or low UHF. Maybe something like 500 MHz as this is the old band IV and there is a sticker on the PCB saying 4 C. If so, it would be a case of testing with a sig gen (set to about -10dBm) and a DVM and tune through maybe 20 MHz to 1 GHz to find the peak in response on the DVM. It will probably be quite unresponsive when not at the tuned frequency so you would have to tune the sig gen in very fine steps if it is a tuned detector.The advantage of having a tuned detector will be increased sensitivity and linearity. The fact it allows external bias suggests that this could have been used as a precision AM or levelling detector at a fixed UHF frequency. It may also allow thermal compensation. The printed inductor and the paler looking shunt cap (near the diode) probably define the resonant frequency and the little trimmer cap provides the match and fine tuning of the centre frequency. The darker looking shunt SMD cap near the ferrite bead will be an RF bypass cap at the output of the detector and it will affect the video bandwidth of the detector.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 20, 2025, 09:42:10 pm
content: One mystery down, this is NOT a biased detector. The device on the back connected to the solder post is a thermistor, connected to ground. Combined with the heavy brass body, temperature compensation makes sense.The attenuator seems to be 3db.From -70 to 0 dbm I see no DC output, anywhere from 0-1400 MHz.Even if its a tuned detector, one would expect _some_ output at lower frequencies, right?

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2025, 09:46:31 pm
content: The diodes in such detectors are fragile--do you know anything of its provenance?

author: G0HZU
author_rank: Super Contributor
timestamp: August 20, 2025, 10:31:50 pm
content: Quote from: ChristofferB on August 20, 2025, 09:42:10 pmThanks. I did suggest it had thermal compensation because the device poking out does look like a thermistor to me.If it is tuned, it could be very narrowband even at 500MHz. It could be maybe less than 10MHz wide at the peak.With modern SMD components it should be possible to make something similar or better for low cost. I’ve made quite a few detectors like this although mine are all wide band, with ultra low VSWR through into UHF and a flat response down to about 100kHz.I mainly use them for levelling sig gens or for basic metrology.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 21, 2025, 09:03:07 am
content: It was sourced from a highly cross-discipline engineering environment under vigorous UV irradiation.(Got it at a HAM market, along with a hideous sunburn)Sadly it was just stored in a PE bag. I assume it was salvaged from old test gear.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 21, 2025, 09:36:02 am
content: Thank you, you may be right, I'll try to resist the temptation of reusing the thing and swapping the diode element until I'm certain it's toasted.I have seen plenty of DIY detector projects, and made a few myself (with the very modern 1N23 diode..) my hope was to get a "known good" reference detector, as I don't yet have the setup to characterise it. Out of interest, what diodes do you use?

author: G0HZU
author_rank: Super Contributor
timestamp: August 21, 2025, 06:55:37 pm
content: I used a LBSD (low barrier Schottky diode) in my detectors. I've had them many years and use them to cross check test equipment and to level signal generators when needed.The main design aim was that the detector should read flat down to about 100 kHz so I could check/cal it using a clean 100kHz sinewave signal measured with a decent 6.5 digit DVM (eg Keithley 2015THD) and then rely on the low VSWR and flat frequency response to take it up into UHF.Detectors like this are prone to significant errors if harmonics are present so the signal source has to have very low harmonic distortion. -50dBc is nice but better than -65dBc is nicer still.If a dual diode is used in the detector this makes it less prone to errors from even order harmonics but this comes at the expense of bandwidth. My dual diode detector is good to about 500 MHz but the single diode detector is good to 2 GHz.I usually check them at 0dBm at 100 kHz and cross check this against the 1mW 50 MHz signal from my power meter and then check it against a thermocouple power meter up through UHF. It's easily good enough for home use and probably OTT even for the vast majority of professional use when it comes to accurately levelling a signal generator for example.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 26, 2025, 12:19:16 pm
content: I might have to try rolling my own detectors then. I have made a few, but they have always been plagued by strange resonances or uneven responses.I have read that impedance matching is difficult for diode detectors - is the approach seen in this detector a decent compromise, to just have a commercial 3db smd attenuator at the input?