RF probe with gain based on ultra cheap S9018 transistor - Page 1
Forum section: electronics / rf-microwave

author: profdc9
author_rank: Frequent Contributor
timestamp: April 19, 2020, 11:07:29 pm
content: The S9018 is this really cheap transistor available from ebay/aliexpress that has a transition frequency between 800 and 1100 MHz depending of the manufacturer.I designed an amplifying RF probe based on it. You can find the github with my schematic at:https://github.com/profdc9/ActiveRFProbeIt uses three stages: an emitter follower for high impedance input, a common emitter for some gain, and a second emitter follower to drive a low impedance cable.It also has a blinkly LED to indicate the presence of RF as well as a demodulator and audio output as well.The probe is designed to be part of the PCB using a cutaway, and there is areas without solder mask that are soldered to for shielding.If there are comments please let me know I would be interested to hear.

author: tggzzz
author_rank: Super Contributor
timestamp: April 19, 2020, 11:29:21 pm
content: What is the measured performance?

author: profdc9
author_rank: Frequent Contributor
timestamp: April 19, 2020, 11:34:49 pm
content: I just designed it a few hours ago. I guess I wanted to see if I had done something stupid before sending it off to JLCPCB.

author: tggzzz
author_rank: Super Contributor
timestamp: April 20, 2020, 01:22:34 am
content: What's the simulated performance?For that matter, what do you want the performance to be?

author: profdc9
author_rank: Frequent Contributor
timestamp: April 20, 2020, 03:09:37 am
content: The purpose of the probe is to provide 3-6 dB gain up to 200 MHz and most importantly not load the circuit. On another forum, someone recommended to me to shrink the board to decrease the capacitance, so I just revved it and made it so a short wire is soldered to the board as the probe. Hopefully the probe, the few pads, and the transistor base (about 1 pF) should stay well under 10 pF load. The emitter follower on the first stage should have to Miller capacitance as the collector is tied to the high supply.The motiviation is that I was looking at designs based on the BF998 dual gate MOSFET and noticed that it is not for use for new designs. The S9018 seems to be a jellybean sourced part nowadays and might be pushed into service as a probe, so the challenge is to see if I can get enough bandwidth out of the cheapo S9018 to make a reasonable RF probe. The S9018 is a SOT-23 version of the SS9018. This is a Sunday afternoon while locked in the house type project.

author: OwO
author_rank: Super Contributor
timestamp: April 20, 2020, 04:07:38 am
content: Try 2sc3357 (2sc3355 if you insist on through hole) which has a 6GHz fT. It oscillates easily though in the through hole form.

author: tggzzz
author_rank: Super Contributor
timestamp: April 20, 2020, 10:37:42 am
content: Quote from: profdc9 on April 20, 2020, 03:09:37 amWhat lower frequency are you expecting?What input voltage range are you expecting?Your input capacitance is higher than desirable: 8pF + board construction.Are the input protection diodes doing anything useful?Be aware of the significantly non-linear capacitance of many ceramic capacitors.

author: profdc9
author_rank: Frequent Contributor
timestamp: April 20, 2020, 03:07:19 pm
content: Ok, I've been doing some work on it. I've ditched the S9018 and put in the SOT-89 2SC3357, which looks like a good alternative, cheap, and widely available. I also changed from BNC to SMA since the bandwidth is likely to go up.I also ditched the input protection diodes. To reduce the input capacitance, I removed the ground plane from underneath the AC coupling capacitor and bias resistor so that the input capacitance is going to be residual capacitance to the ground plane not under these components adding to the base capacitance. I was considering using a 100 pF @ 630 V or 1 kV input 0805 ceramic as the AC coupling capacitor. I calculate that the input impedance of the BJT base should be approximately hfe X 220 ohms or about 22 k ohms (excluding parallel capacitance at high frequencies), so that the lowest usable frequency should be around 200 to 250 kHz. I also crammed all the bias and feedback components in as close as I could to the transistor to minimize inductance.I am looking for a small signal type RF amplifier (up to a volt or two input) that will minimally load the circuit.Thanks for the suggestions. How about now?

author: StillTrying
author_rank: Super Contributor
timestamp: April 20, 2020, 03:48:52 pm
content: I'm not really following this probe, but a capacitor driving just the end of one diode is usually not a good idea.

author: OwO
author_rank: Super Contributor
timestamp: April 20, 2020, 03:51:19 pm
content: It looks like you are fine with SMT, so if you want even more bandwidth/lower input capacitance also consider the BGA616 ($0.2-0.4), which has the advantage of near-flat gain up to 1GHz. The downside is the higher voltage required (6V for highpass bias-tee, higher for pure resistive bias).You can put a resistive divider at the input side to raise the input impedance further; the gain of the MMIC will make up for the loss of the divider.

author: profdc9
author_rank: Frequent Contributor
timestamp: April 20, 2020, 05:07:50 pm
content: Ok, I realize the mistake of having both capacitors in the peak detector circuit. That was a leftover from the earlier incarnation. Oops.I like the 2SC3357 that you suggested because it's widely available and there are generic Chinese suppliers. One of my goals here is to make something that will not be obsolete soon because the other do-it-yourself designs I have seen use the BF998 dual gate MOSFET which is not recommended for new designs. Are there any other transistors with a higher transition frequency that are generic like the one you suggested?I am not great at SMT assembly though I can do it. 0402 is getting beyond my coordination and eyesight, and 0603 is ok but pretty ugly when I do it. But the 2SC3357 is also available from JLCPCB's SMT assembly process, so the whole design except for the SMA connector can be populated by JLCPCB inexpensively. Nevertheless, I stuck with 0805 components just to keep things big enough for klutzes like me to do by hand if necessary. I also recognize that the bandwidth will be horrible without SMT.

author: cjheath
author_rank: Contributor
timestamp: April 22, 2020, 12:57:34 am
content: I did a project like this a few years back - but I wanted a high input impedance. You can see the design here:http://polyplex.org/electronics/RFCascodeProbe/I was disappointed in the performance (flat to only 350MHz) until I got a response from Winfield Hill (he of "Art of Electronics" fame) who pointed out that the low Vce of the upper cascode transistor means it has an unexpectedly high output capacitance. That, combined with the 220R load, determines the 350MHz roll-off.I mean to revise this design to increase supply voltage to 12 or 15V, and decrease that resistor to 100R or less, and that should get to 700 or 800MHz.The tip design using a short fixed pin beside a pogo ground pin means there is a very small loop area, which improves performance. To go better, the input components should be dropped smaller than 0805.If you can't get the BF862, there is a very similar JFET still in current production, the CPH3910

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 22, 2020, 01:31:40 am
content: Hi,Let me contribute an LTspice model of your circuit:Modelling ResultsIt looks like it might workYou might want to decouple the power rails on the real schematic with some 10resistors between the stages.(You are showing PNP transistors on your schematic. 2SC3357 are NPN)I have attached the LTspice model.Regards,Jay_Diddy_B

author: OwO
author_rank: Super Contributor
timestamp: April 22, 2020, 03:48:57 am
content: A FET based probe loads down the input too much at high frequencies (BF862 has 10pF input capacitance which is huge); I think the bipolar/MMIC approach + input divider has more potential and I'm not sure why everyone still seems to go with FET probes.

author: cjheath
author_rank: Contributor
timestamp: April 22, 2020, 08:00:06 am
content: Well to be fair you can use a 10:1 attenuator (R parallel with C) to reduce the BF862 input capacitance to 1pF - since you have gain following. And the other mentioned FET is roughly half the size of the BF862.You can't get such good noise performance from smaller FETs, so it's horses for courses.

author: profdc9
author_rank: Frequent Contributor
timestamp: April 22, 2020, 03:10:39 pm
content: That's pretty awesome!Thanks so much! I actually did my own model in QUCS (quite universal circuit simulator yesterday and got a similar result). I put it in the zip file.I simulated the input up to 1k source resistance at 1 GHz. Seems to work even with that.I was so encouraged by the result I wanted to take my hand at doing a differential probe based on the same idea, so here it ishttps://github.com/profdc9/ActiveDifferentialProbeBecause JLCPCB offers a SMT assembly service, I have added the LCSC part #s to the schematic. This should make it really easy to have these assembled.

author: profdc9
author_rank: Frequent Contributor
timestamp: May 18, 2020, 04:27:54 pm
content: Here is are the results for the gain of the RF probe.The performance seems to be better than expected. I had to make a few tweaks to the design as shown above, mainly adding 22 ohm base resistors to the emitter follower stages.The plans are in the githubhttp://www.github.com/profdc9/ActiveRFProbeThanks for taking a look!

author: fmkit
author_rank: Contributor
timestamp: July 07, 2020, 09:49:05 pm
content: easy way to make an RF probe - convert GPS LNAI've bypassed 1.56-1.60GHz SAW filter and added 20ohm resistor on input pickup loop to stop oscillation.Usable range 176MHz to 6GHz