TRF7960 impedance match

Mark Schwab

Other Parts Discussed in Thread: TRF7960

The TRF7960 reader's output can either be 8 ohms or 4 ohms depending on the input voltage. Should the matching components be changed when switching from 5 volts to 3.3 volts? I can only find literature on the 4 ohm output match. I'm running it at 3.3 volts so I matched the output to 8 ohms. I would think that if the reader was operating from 3.3 volts with an 8 ohm output the match would have to be changed for maximum efficiency.

Also, I've designed an external class e amp and everything looks good except the voltage that would be at RX1 and RX2 is in excess of 3.3 volts. It's roughly 5-6 volts. I'm planning to attenuate the signal, but I want to know if the RX1_in and RX_2 in pins have protection diodes.

Thank You

over 8 years ago

0 Josh Wyatt over 8 years ago

TI__Guru* 90906 points

the output impedance is driven by the Chip Status Control register.
this means that at full power the output would 4 ohms and half power is 8 ohms.

what power level are you trying to reach?

0 Mark Schwab over 8 years ago

Prodigy 235 points

Half power. That means I need a match for 8 ohms right?

On Wednesday, July 29, 2015, Josh Wyatt <bounce-614@mail.e2e.ti.com> wrote:

A Message from the TI E2E™ Community

RE: TRF7960 impedance match

Josh Wyatt

the output impedance is driven by the Chip Status Control register.
this means that at full power the output would 4 ohms and half power is 8 ohms.

what power level are you trying to reach?

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Mark Schwab

Prototyping Engineer

Schwab Electronics Labs - From paper to Prototype

http://www.schwabelectronics.com

mark.schwab@schwabelectronics.com

(402)937-9727

0 Josh Wyatt over 8 years ago in reply to Mark Schwab

TI__Guru* 90906 points

yes - but they would you run at lower voltage and half power, then amplify it?

0 Mark Schwab over 8 years ago

Prodigy 235 points

I need about 500mW. The mosfet I'm using doesn't require much gate drive so I don't need to force 200mW into it. I can't get into specifics, but I have the gate of the fet (127 ohm) matched to the reader's output (8ohm). I was wondering why if at half power it switches to an 8 ohm load but I can't find anywhere in TI's documentation where they use an 8 ohm impedance.

0 Josh Wyatt over 8 years ago in reply to Mark Schwab

TI__Guru* 90906 points

Mark -

in the TRF7960 datasheet () see page 8 you will see parameter: RRFOUT (PA driver output resistance) listed

if you send me email at josh.wyatt@ti.com i can give you known good design that can do what you want, also Class E amp.

0 Mark Schwab over 8 years ago

Prodigy 235 points

It shows the impedance match to the 8 ohm output in the datasheet?

0 Josh Wyatt over 8 years ago in reply to Mark Schwab

TI__Guru* 90906 points

no it lists it and the condition (i.e. being set to half power)

here are the matches (using standard values) i have just done for you from 8 ohms out, using both single and dual receivers - see if these help you at all.

single

Dual

0 Mark Schwab over 8 years ago

Prodigy 235 points

That looks right. Thank You

I'm trying to figure out the RX1 and RX2 input pins. The 790pF capacitor above would be 2 capacitors in series and the RX1 pin would be tied in the center of the two capacitors. I'm assuming this is to decrease the voltage to the RX1 pin. Given the case above what values would you recommend for the 790pF capacitor? The 790pF cap needs to be a series combination of two capacitors and then where they join goes back to the RX1 pin.

Thank You

0 Josh Wyatt over 8 years ago in reply to Mark Schwab

TI__Guru* 90906 points

two 1600pF in series probably close enough

0 Mark Schwab over 8 years ago

Prodigy 235 points

That's my dilemma. I'm amplifying the signal so my peak to peak voltage is about 15 if I'm applying 3.3 volts to the chip what ratio should I have those capacitors at? Also can I use a single receiver if I'm only using the chip for tagit and iso14443? Those should be my last questions

Thank you

On Friday, July 31, 2015, Josh Wyatt <bounce-614@mail.e2e.ti.com> wrote:

A Message from the TI E2E™ Community

RE: TRF7960 impedance match

Josh Wyatt

two 1600pF in series probably close enough

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Mark Schwab

Prototyping Engineer

Schwab Electronics Labs - From paper to Prototype

http://www.schwabelectronics.com

mark.schwab@schwabelectronics.com

(402)937-9727

0 Josh Wyatt over 8 years ago in reply to Mark Schwab

TI__Guru* 90906 points

50% ratio works here generally - in your case you may need to add dampening resistor in series before the RX pin - i recommend measuring it before connecting it.

also, 2200 and 1200 get you close (~776pF) and would provide similar ratio as we see used to dampen RX_2 input on the EVM

0 Mark Schwab over 8 years ago

Prodigy 235 points

I have it working fairly well now. I'm reading multiple tags from a short distance. When the tags are within an inch of the antenna the RX voltage drops to around 3vP-P when I take the tags away the voltage goes up to 5vP-P Can the RX inputs handle that reliably or should I add some protection?

Also I measured the antenna using MFJ-259 antenna analyzer and it looks like the antenna goes from 0+127j unloaded to 30+30j when I load it with tags. I matched the antenna using the 30+30j impedance. Do you think this would be a typical result using 10 tags on top of the antenna? Thank You

0 Josh Wyatt over 8 years ago in reply to Mark Schwab

TI__Guru* 90906 points

when the antenna is matched correctly, it will be 50 +j0, at 13.56MHz and have a bandwidth of 848kHz.

using the MFJ, this means that (once you have it centered on 13.56MHz, where SWR = 1, R= 50, X=0 (or close to it)) you would run the frequency adjust knob down to 13.136MHz and up to 13.984MHz, and look for the SWR to go to 2:1 to know your BW is correct (which also means your Q is correct.)

0 Mark Schwab over 8 years ago

Prodigy 235 points

Because this application has multiple tags very close to the antenna should I place the tags near the antenna before I do the measurements? I can tune the antenna without the tags but once tags are near to the reader's antenna everything changes.

Thank You

0 Josh Wyatt over 8 years ago in reply to Mark Schwab

TI__Guru* 90906 points

of course - these are resonant circuits. Please tune it as i suggested.

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TRF7960 impedance match