RF choke selection in 4-7GHz monolithic amplifier GALI-39 - Page 1
Forum section: electronics / rf-microwave

author: analogRF
author_rank: Super Contributor
timestamp: June 13, 2020, 09:15:11 pm
content: This is actually with regard to repair of a spectrum analyzer but it can be a general design questionIn the instrument I am currently repairing, there is a GALI-39 amplifier used in the LO path of the first converter and the LO is from 4GHz to 7GHzThe amplifier is biased from a constant current source (not by a resistor connected to Vcc). Basically a PNP transistor provides a current source and the collector is connected to the output of the GALI-39 through an inductor (RFC)Now the GALI-39 is dead and in the process of removing it I manged to destroy the closely spaced inductor (RFC)of course there is no schematics or parts list available and I need to replace the inductor.It is a wire wound inductor in 0603 package and I can clearly see it has 2 turns of wires and it has a color dot which is either orange or brown, I cannot tell.I am assuming the self resonance freq of the choke has to be above 7GHz. Am I right?on the other hand the reactance of the coil must be much larger than 50ohm at 4GHz.Above two criteria make it impossible to find a suitable replacementI searched through Digikey and Mouser, if I set the self resonance above 7GHz, the highest inductance I can get is 5.6nH or 4.7nHwhose reactance at 4GHz is 140 and 118 ohm, respectivelyany ideas are appreciated.

author: ch_scr
author_rank: Frequent Contributor
timestamp: June 13, 2020, 10:12:24 pm
content: Have another choke in series with a higher L and lower SRF towards the DC path? I think TheSignalPath showed a very wideband Bias-Tee made that way on his Channel.

author: analogRF
author_rank: Super Contributor
timestamp: June 13, 2020, 10:34:46 pm
content: there is no space to modify the circuit. I only have one space for a 0603 chip inductorbasically my question is since this thing is being biased by a current source (looking into collector of the PNP current source)is it enough to just have a 5.6nH inductor?I tried to calculate the approximate inductance of the coil considering the dimensions and the fact that it has two turns of wireand the spacing between the wires seems to be roughly about 0.15-0.2mm (used an online calculator) and it seems the inductanceis actually around 3.8-4.7nH !

author: Bud
author_rank: Super Contributor
timestamp: June 13, 2020, 10:49:45 pm
content: I'd think there would be no requirement for the choke to have >>50 Ohm impedance across the working frequency range, since the circuit is biased from a current source. A current source has high output impedance, so the choke will not appear in parallel with the RF load in the equivalent circuit and would not shunt it if its impedance is comparable to 50Ohm. The choke impedance still have to be high enough to be useful though (filter out harmonics and stuff).

author: analogRF
author_rank: Super Contributor
timestamp: June 13, 2020, 11:16:08 pm
content: Quote from: Bud on June 13, 2020, 10:49:45 pmthat's what I have been thinking, too. However, the output impedance of the current source at 4-7GHz might be very low indeed. The PNP transistor used is BC807-40LT1G. Its output capacitance (I think the datasheet means C_{cb}) is more than 10pF. At the bias voltage that I have, its close to 15pF, so the impedance at 4GHz is almost like a short

author: analogRF
author_rank: Super Contributor
timestamp: June 13, 2020, 11:22:54 pm
content: I looked at Coilcraft 0603 inductors for color coding and if I assume the self resonance must be above 7GHz, then the only choices for Brown or Orange color dot are 3.9nH and 4.3nH.Now I have no idea if inductors in this instrument are from coilcraft but they look similar to their picturesI also found another vendor (forgot which one) which had the same color coding with the same resultso it seems that something around 3.9-4.7nH must be good if the SRF is above 7GHz but then the impedance is pretty low compared to 50 ohmunless the SRF can be had within the band? is it ok to have a parallel resonance (very high-Z peak) within the 4-7GHz band?several application notes for monolithic amplifiers from Skyworks and Mini Circuits and Avago tell me that the SRF of the choke must be above theband of interest

author: analogRF
author_rank: Super Contributor
timestamp: June 14, 2020, 02:31:23 am
content: I just quickly put together this simulation of the impedance of the current source + inductor as seen by the GALI-39 output.I used a 4.3nH 0603 inductor from Coilcraft with SRF=7.5GHz (that translates to a shunt capacitance of 104.72 fF)I know this simulation is not perfect and there are still some RF effects/parasitics that are not included but it should not be far off.as you can see in the plot, the impedance is purely inductive and starts from 148 Ohm and goes to about 1460 Ohm.This is basically the impedance of the inductor alone because from the collector of Q1 I have a large 13-14pF capacitor which is basically a short toground.I suppose it is not that bad after all. I think the gain will drop a bit at lower frequencies where the GALI actually has more gainand it will not affect the gain at higher frequencies, so it may actually kinda flatten the gain of GALI-39. I have also attached Mini Circuit graphsfor GALI-39what do you guys think?

author: analogRF
author_rank: Super Contributor
timestamp: June 17, 2020, 03:35:26 pm
content: I did some more simulations after downloading the S parameters for the GALI-39 amplifier (tried both at 35mA and 42mA).Here are just a few samples of results and I am pretty sure now that not only a 3.9nH will work perfectly, even a lower value is still okwhat is more interesting and a bit puzzling to me is that even using an inductor with SRF within the band (4-7GHz) still does not cause much trouble and seems to work!even though all application notes insist on having SRF above the band of interest and that is what I have always used myself in the past when I built amplifiers with RF gain blocks (several below 3GHz and one at 12.5GHz)1-first response is for 3.9nH inductor with SRF=7.5GHz. I used typical values of Rdc and Q from Coilcraft datasheets and playing around with themdoes not make much difference (very little) unless you choose really low Q like 1 or 2 or 5 for example.***This is actually the inductor that I suspect is used in this board that I am repairing***2-second response I changed the inductor to 2.9nH and as you can see results are stilll very good even though the reactance of this thing is just 73 ohm at 4GHz. I even tried 1.5nH (see response 5)3-I lowered the Q to 10 and still not much difference4-I set the SRF of the inductor to 5GHz (it becomes capacitive after 5GHz) and as you can see the response is still not badin some cases I could actually get better matching when SRF was within the band!I have checked the stability of the whole circuit and it is ALWAYS in all cases unconditionally stable over the whole bandI know I have not included the PCB traces (I dont have any info on them) but they are extremely shortand components are very tightly soldered, and I really dont think it will have much of an impact on the response.

author: RadioNerd
author_rank: Regular Contributor
timestamp: June 18, 2020, 10:46:59 am
content: I would expect any decent wire wound inductor in the range from 2.2 to 6.8 nH or even more to work just fine.Dont be too afraid about the (first) SRF, as it is actually even the theoretical ideal operation point (parallel resonance = very high impedance) for a bias teeusing an inductor close to its SRF is however not a very good idea in general because its behavior in that range is 1. strongly frequency dependent, 2. not well defined and 3. varies quite strongly with manufacturing tolerances.The real killer for biasing applications are series resonances in the biasing circuit as they lead to nasty dips in the frequency responce of the amplifier