Cascade RF Frontend 2.4GHz possible?

[Flunze]

I have a fritzbox to repair and the original FEM is hard to get, also I got a ton of RFX2401C lying around, so I want to use those. Without this I get -65dbm and with about -40dbm (deadbug soldered in place) Edit: This is perfectly within spec, so not my messy circuit, the RFX2401C is supposed to have 25db gain - so perfect. But -40dbm as the highest power is not enough, it should be around -20dmb, so I need another stage. Is it possible, to just put the TX-Enable in parallel and the Output of FEM1 to the input of the FEM2? RX-Enable is always high and it does not matter, as TX-Enable overrides it in priority, which makes it much more easy. It is not supposed to be perfect and I still need to dead-bug solder the small drop in PCB, but that is much more easy than 2 ICs hanging in the breeze. I thought of this layout (I already refined it, but the principle and overall layout is identical) - see attachment.


Will the FEM just blow up or should this work? The input and output filter stage will be used from the original design. I also do not care for harmonics - it is supposed to be used in a steel concrete bunker and even with -20dbm the range is quiiiite limited. So no harm to others to consider.

I was so crazy as to solder 2 in series deadbug style, took 3 hours and than I missed a short on the Stage 2 output. Now the power is down again to -65dmb. I already know what it means, both Front End Modules are dead for sure. That is, why I wanted to make my life easier with a simple PCB. And I enjoy it. It is not supposed to make perfect sense, so do not worry, if it sounds a bit overcomplicated for the purpose.

[Flunze]

Please see attachments for Front, back side and the circuit I want to use.

[vk4ffab]

Please see attachments for Front, back side and the circuit I want to use.

My gut says, nope, bad idea. The RF in and Antenna ports have maximum ratings of 5dbm into them and you want to blast almost 1/2 a watt into it. The output power from one of those chips should be 22dbm. BOOM dead chip. Just buy a new router.

[Flunze]

Hi vk4ffab. Thanks for your reply. But I think you overread something:

Without this I get -65dbm and with about -40dbm

This means the Backend outputs around -65dbm and the first stage -40dbm. I mean, if I would have +5dbm why would I bother amplifying anything? I have no way to directly measure this however. It is possible, that my phone is not really the best. But the manufacturer claims at most 100mW of output power (consisting of 4 channels). Originally I measure however -20db. Okay lets do some simple math:

The 20dbm in total for a single channel are 14dbm for the original circuit. My phone measures -20dbm (a lot of loss, but very repeatable measurements with multiple devices). Okay, I measure -40dbm with a single of the replacement Frontend IC's. That is 20db lower, so +14dbm-20db = -6dbm. This is well within range of what the input of these ICs can handle according to the spec.

Okay, this clearly shows that the -65dbm at the beginning are incorrect, but it backs up that I have 1 order of magnitude of safety left. That also means, that if it works, the output power basically lands right in the perfect spot of +20dbm. That would actually be absolutely amazing.

My question is just, if you look at the circuit: Can you chain 2 of these ICs in series, or do you need some passives in between? Remember, I do not look for perfect performance at all. It just needs to work reasonable for a low effort. I would think, that the Output is basically at DC level, just like the input - so I would think it should be fine. All circuits use 3.3V as input, so I expect the voltages to be at equally fine levels.

TLDR:
- Input power is definitely within safe range
- Question: Do I absolutely need some passives or could a direct ANT Out to RXTX work with a direct connection? (ignore impedance and parasitics for now)

[vk4ffab]

Hi vk4ffab. Thanks for your reply. But I think you overread something:

Without this I get -65dbm and with about -40dbm

That chip should put out, on TX P OUT = +20 dBm  its in the datasheet. Whatever drives it should be giving more power than -65dbm, I would not be trying to amplify a poor signal, i would be finding out why you have so little signal to begin with.

WiFi typically puts out between .2 and .5 of a watt. So lets work backwards, you should have 23dbm at 0.2w, minus 25dm gain typical in the datasheet for that chip, equals 1dbm drive, or 1.26 milliwatts. Orders of magnitude more power than -65dbm drive you have. Something is loading it down for the drive level to be that low.

FWIW, i am not a microwaves and wifi guy. Not an expert, just a ham who builds his own HF radios. But my spidy sense says its something else that is the problem.

[Flunze]

@vk4ffab

I have 15 of these boxes with exactly the same output power. And yeah, it likely is not -65dbm drive as from my aproximation.

The wifi of these boxes does indeed not work with 0.2 or 0.5 Watts. This is illegal in germany. There is also nothing that can load the signal down. I mean there are 4 identical RX/TX Pairs per Fritzbox 7590 and I have 15 behaving the same. I just think, that the proprietary 055F has a higher gain. This is backed up by the fact, that some people replaced this IC and got the full range back. There is no short. It is just a higher gain in this case.

Please just ignore this gain nonesense for now.

I only want to know, if such an IC can directly drive such an IC or if passives are absolutely required. That is all I asked for. Let the gain be my problem.

[vk4ffab]

@vk4ffab


I only want to know, if such an IC can directly drive such an IC or if passives are absolutely required. That is all I asked for. Let the gain be my problem.

Single-ended 50 Ω input and output ports.

^^ from the datasheet, 50ohm in, 50ohm out, I could check that there is no dc on either the in and out ports, if there is it would require a blocking cap, other than that, as long as the in and out ports do not see more power than they can handle and you can sequence the switching from tx to rx accurately enough so they both have fully switched before going into TX, because you do not want the switches being hit with highish power rf while switching, then sure, I cannot see a reason why it cannot work, other than you now have 2 LNA blocks in series on receive and its impossible to say what that would do to receiver performance. The receiver getting overloaded could be very detrimental to its performance.

So they answer is Yes, No and Maybe? In TX I would say yes, on RX I would lead towards no because the 2nd lna is likely to distort when the first LNA has a strong signal and maybe the receiver can deal with strong distortion, but probably cannot. You can always just do what most hams do and just give it a go and see what happens, but, what you really need is a gain block with switches before your IC that does not have an LNA on rx. I do not know if such a thing exists.