Series connection of GAIN BLOCKS - Page 1
Forum section: electronics / rf-microwave

author: Maksims
author_rank: Newbie
timestamp: December 27, 2023, 12:37:40 pm
content: Hello everyone.First of all I have to say, that I am not very familiar with RF circuits and so, if I say something very stupid, please understand.But, I will try to explain as more details about my idea, as possible.I have the following design: MAX2750 VCO coupled with ADL5611 gain block. Frequency range : 2.4 - 2.5 GHz. PCB is made on ROGERS substrate and is etched by hand. This is only hobby project, so is is not worth to order ROGERS or PTFE PCB, since it will be very expensive.This design works very well. But, I would like to increase the output power. I think, this can be done by connecting more gain blocks in series, for example, ADL5611, followed by SKY65162. ADL5611 has higher gain, but less output power. SKY65162 has lower gain, but higher output power. Parameters of the used parts in specified frequency range can be summarised as following:MAX2750. Output Power: -3 dbm.ADL5611. GAIN: ~21 db. Output power (OP1): ~ 20 dbm.SKY65162. GAIN: ~13 db. Output power (OP1): ~29.5 dbm.If all this stuff is connected in series, output power, theoretically, will be equal to : -3 + 21 + 13 = 31 dbm. A bit higher, than OP1 of the SKY65162, but not dramatically. Also, real gain and, as a result, output power will be a bit less, because of non-ideal PCB design.Typically, inputs and outputs of gain blocks are capacitively coupled and recommended capacitor values are specified in datasheets. So, when recommended circuits for both gain blocks are combined in series connection, the following circuit is produced:So, my main question is about the part of the circuit, where output capacitor of the ADL5611 (100pF) is connected to the input capacitor of the SKY65162 (0.8 pF). Should I keep both capacitors (100 pF + 0.8 pF)? Or should I keep only 0.8 pF, since it is series connection and smaller capacitance is dominant in this case? Or, maybe, some other option?

author: A.Z.
author_rank: Super Contributor
timestamp: December 27, 2023, 01:24:58 pm
content: just eyeballed the datasheets, so please bear with me; the sample for ADL5611 is designed for 50 Ohm input and output impedance, not sure about the "sky" input impedance, at any rate I'd consider using a matching network between the two stages, and possibly one acting as a lo-pass filterjust sayin'

author: tszaboo
author_rank: Super Contributor
timestamp: December 27, 2023, 05:42:31 pm
content: The IC's are internally matched to close to 50Ohm. You don't need to place two capacitors in series, at 2.4GHz 10pF is already ~0Ohm. The last IC would be driven beyond it's compression point, and it would spew out a lot of harmonics. You don't accidently loose 2dB on a PCB with well matched circuit. If you need to attenuate, you need actual resistors for that.Just an FYI, it would be also super illegal to connect this to an antenna. At the 2.4GHz band, your TX power is limited to some 13dB or so, depending on your Antenna gain, on time and so on.

author: eb4fbz
author_rank: Regular Contributor
timestamp: December 28, 2023, 02:00:52 pm
content: You can remove the 100pF cap at ADL5611 output. That MMIC is internally matched to 50ohm. You must however replicate the SKY65162 input and output matching.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 28, 2023, 05:15:46 pm
content: The MMIC with the highest Output power should be the final.

author: Maksims
author_rank: Newbie
timestamp: December 29, 2023, 01:40:23 pm
content: Thanks, everyone. As i understand, i can do it and see, what will happen. Maybe some adjustments or matching will be necessary.Quote from: Wallace Gasiewicz on December 28, 2023, 05:15:46 pmYes, of course. I understand this.Quote from: eb4fbz on December 28, 2023, 02:00:52 pmQuote from: A.Z. on December 27, 2023, 01:24:58 pmOkay, I will experiment.Quote from: tszaboo on December 27, 2023, 05:42:31 pmIt would be much more illegal, if the VCO is modulated with sawtooth wave, while all this is connected to an antenna.

author: LaserSteve
author_rank: Super Contributor
timestamp: December 30, 2023, 03:04:31 am
content: Generally, we pad mimic gain stages with 1 dB PI attenuators if unsure of the matching. This leaves a means to reduce drive by changing 3 parts on the board layout if suddenly end up in overdrive or compression.Steve