The Purpose of RF Choke (Inductor) in MMIC Circuit and Its Proper Value - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 22, 2018, 06:56:33 am
content: i bought some mmics and their respective rf pcbs for learning and i got stumbled with deciding which inductor value is most suitable for what. the bandwidth i'm planning is near DC (few KHz) up to 5-8GHz, just a simple gain RF amplifier. for ERA-1SM+, the RF choke is said to be "optional" (attachment 1) though recommended in the datasheet to gain some 1+ dB amplification (from what i understand), so meaning i can only use a 0 ohm resistor in place and loss 1dB, correct? and then for HMC480, there is a table of recommended variety of rf choke (attachment 2) for each working frequency, and thenhttps://www.changpuak.ch/electronics/mar_era_bias.phpERA component calculator gave me different value. say at 5GHz, HMC480 recommending 6.8nH, but ERA app says 16nH, so i got confused. SOS please, i try to buy some inductors here (0603 ceramics if that matters) i dont want 2 weeks from now i figure out i got the wrong value and have to order and wait again and left me with some useless components. waiting your reply thanks

author: awallin
author_rank: Frequent Contributor
timestamp: March 22, 2018, 07:09:53 am
content: "DC-to-daylight" as a requirement is tough! You'll have to build a prototype and see how far up in bandwidth it goes - and if you get any oscillations etc.The choke blocks the signal itself from going up to the Vcc supply and being absorbed by the (large) DC-coupling cap.Finding a choke that works well from DC to 8 GHz is going to be hard.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 22, 2018, 08:57:54 am
content: can i just put a big enough inductor so low freq say < 1MHz will be absorbed hence any freq greater than that will pass? from the app, it will be about 80uH

author: tggzzz
author_rank: Super Contributor
timestamp: March 22, 2018, 09:07:39 am
content: The parastics in the inductor will determine its effectiveness.You would benefit from modelling the output circuit in Spice, including parasitics. Then vary the parasitics' values from negligible, through sensible, to ridiculous - and see how the circuit's behaviour changes.Then you can build something, measure it, and have a starting point for what's wrong and how to improve it.Determining the parasitics is best done by looking at the datasheet. Decent RF inductors will have sufficient information, others won't.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 22, 2018, 09:31:26 am
content: Quote from: tggzzz on March 22, 2018, 09:07:39 ami'm thinking of just buying some cheap 0602 smd ceramics inductor and characterise it later with VNA or something. i'm not sure if i can correctly model the parasitics and the mmic in simulator. the choice of 0602 footprint because the pcb i bought only provide that, bigger inductor wont fit. as a "descent RF grade quality" inductor, i'm not sure where to look for, digikey or anywhereonline is too expensive in term of shipping if i just want to buy few cents of components. any suggestions?

author: tggzzz
author_rank: Super Contributor
timestamp: March 22, 2018, 11:38:33 am
content: Quote from: Mechatrommer on March 22, 2018, 09:31:26 amModelling parasitics is difficult at the best of times, and will never be completely correct. At best simulation can give hints of what happens in a physical circuit, to help which parameters are more/less important.Practical inductors are particularly complex "circuit elements": non-linear and very dependent on construction and materials.Nonetheless, if the manufacturer gives no information your only alternative is to measure devices and infer your own models. That is far from uncommon at RF/microwave frequencies, where the PCB material has significant effects on performance.You might benefit from talking to radio hams, and/or getting an old copy of the ARRL Handbook. The fundamentals don't change much over time

author: Performa01
author_rank: Super Contributor
timestamp: March 22, 2018, 02:04:11 pm
content: From the two application circuits shown in the opening posting, only the first one can be recommended.The 2nd one cannot go down to low frequencies unless L1 inductance is very high, which would make it unusable at high frequencies in turn.Then the (unlucky) combination of L1, C3, C4 and C5 looks like a good idea, but usually isn’t in practice, because it forms a multi-resonant circuit that might cause all sorts of troubles at high frequencies, depending on the individual component properties and the mechanical construction of the circuit.The first application circuit might provide slightly less gain, but has significantly wider bandwidth and is much more benign. The resistor Rbias has to be rather low to provide the required (high) bias current, hence it causes additional load at the amplifier output, which in turn will reduce the gain especially at high frequencies. This is why the optional RFC can be added, so that the amplifier only sees the actual load connected to the out terminal at high frequencies. The inductance should be optimized so that the gain is as flat as possible, which would be hard to predict in advance, because there are so many variables. It has to be tested in the actual circuit. For instance, the Resistor Rbias will have some inductance already, but also some capacitance, which makes it a parallel resonant circuit and its resonance frequency should be above the upper bandwidth limit of the amplifier. Yet it might be just right and that’s another reason why RFC is optional.

author: dmills
author_rank: Super Contributor
timestamp: March 22, 2018, 03:17:53 pm
content: Yup, DC - 8Ghz is hard, possibly unreasonably so if you don't know exactly what you are doing.I would go for the resistive termination in the first instance, 1dB nearly never makes much difference when it is gain as opposed to say noise figure, and the less things you have that go resonant in your operating bandwidth the better.SRF is the figure of merit in inductors for this kind of thing and you want the SRF to be greater then the operating bandwidth (not easy at 8GHz), usually you wind up putting a few inductors in series to cover the bandwidth, one low value with a high SRF then a higher value with lower SRF and such, often a resistor across the inductors helps kill excess Q.Regards, Dan.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 22, 2018, 03:46:56 pm
content: it seems i still have to buy few values to try on. thanks guys.