Series connection of GAIN BLOCKS

[Maksims]

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?

[A.Z.]

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 filter

just sayin'

[tszaboo]

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.

[eb4fbz]

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.

[Wallace Gasiewicz]

The MMIC with the highest Output power should be the final.

[Maksims]

Thanks, everyone. As i understand, i can do it and see, what will happen. Maybe some adjustments or matching will be necessary.

The MMIC with the highest Output power should be the final.

Yes, of course. I understand this.

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.

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 filter

just sayin'

Okay, I will experiment. 

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.

It would be much more illegal, if the VCO is modulated with sawtooth wave, while all this is connected to an antenna. 

[LaserSteve]

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