Minicircuits ZFL-500 directivity question

Or rather |S12| - |S21|

It's just the difference in dB, from the datasheet s21 is about 25dB, directivity about 38db, so s12 is 38dB below s21, or -13dB.

That's wrong - would be -(25+38)dB

Thanks for your reply but I don't quite follow - do you know how they calculate the ~40dB figures of the top table on page 2?

It is just the magnitude of the amplifier isolation (reverse gain) minus the (forward) gain. If reverse gain was -80dB and forward gain 20dB direcivity would be 60dB.

Gotcha, thanks!

Isolation is 1/(reverse gain), and it’s a straight subtraction, so 60 dB is correct for the wrong reason.

One way of keeping it straight is that an N-dB pad has 2N dB isolation. (Using the magnitude would predict an isolation of 0 dB.) The idea is to say how much a mismatch affects the input impedance.

Cheers

Phil Hobbs

Thank you, so just S21 - S12? S21 for the ZFL-500 was about 20 dB and S21 -40, that would seem to imply 60 dB directivity while the spec says around 40 in the mid band, seems hard to believe it's beating the spec by that much.

But I'm unsure how much input power affects this, does the datasheet table imply these measurements are taken at the 1dB compression point?

Sorry S12 was about -40, rather

To clarify I mean the log magnitudes here, as a VNA so conveniently can display for us

A reverse gain of-40 dB is an isolation of +40 dB, so the directivity is

If you put a short or open on the output, it will reflect the full output power back into the output port. That’s 20 dB above the input power, so assuming a matched input and 40 dB isolation, you’d get an input return loss of 20 dB. That’s what the directivity expresses.

Cheers

Phil Hobbs

I see why JL loves his time domain.

The 5080 is a nice instrument, it seems to boot to Windows 7. Crashes to Windows 7, too..

They give S-params for 2 units. Conveniently, they are in dB/angle. The table just says 38 dB, more or less for directivity. By a quick look at the S-params and comparing to the datasheet table, I think they are just saying D = |S12| (dB), which you noted.

Example: For "unit 1" and at 100 MHz, forward gain is +24.47859 dB. For "unit 1" and at 100 MHz, reverse gain is -38.36183 dB.

Because the "directivity" looks to be about same as |S12|, it is safe to say they mean D = |S12| (dB)

!Freq S11:SOLT2(ON) S21:SOLT2(ON) S12:SOLT2(ON) S22:SOLT2(ON) # Hz S dB R 50

I should say: D = -|S12| (dB)

Thanks Simon, yes that seems right, this amp was bang on within spec otherwise so I'd have been surprised if it was way off there.

They're nice lil modules, I didn't have this purpose in mind but I wonder if they make a LNA suitable to replace the s*it ass front end on my DTV antenna, I live in a fringe area so reception varies by the day. and I'm in a condo so mounting stuff outdoors not really feasible

It doesn't?

all of which intended for satellite reception are LN-A's. The customers pay for everything, do you think it's wise then for them to provide junk performance parts making customers into ex-customers?

Could apply the same argument to many other industries...and conclude all products are high-quality because otherwise the manufacturer might lose customers.

Hey, I have a cool idea, I'll see if I can put it on the 5080 next time I have a chance and see how it compares to a Minicircuits gain block for similar bandwidth, I think I can get a hold of one..

It isn't the absolute value in decibels. The directivity is just the magnitude of S21*S12. Otherwise you get the wrong answer for attenuators.

Cheers

Phil Hobbs

That ZFL-500 is not very suitable as antenna amplifier for DTV. First of all it is quite noisy, noise figure NF = 4.2 dB (typical). These days a good antenna amplifier the NF should be below 3 dB. That amplifier is specified only up to 500 MHz, thus most UHF channels are out of range. However, it might still work since the specs are good at

An antenna amplifier should be installed _at_ the antenna, thus the cable between amplifier and DTV receiver can be long.

You seem to suffer from tropospheric fading and the signal nulls can be quite deep and the signal is still momentary lost no matter what antenna amplifier you use. Only a proper outdoor antenna will really help.

Yeah it has gain for days. S21 is above 20dB well past 1 GHz.

I'll see if high pressure systems seem to make it better...atm with a planar antenna and associated amp I'm getting a station with a purported field strength of -70 dBm at 36 miles no problem if I kinda point it in that direction, but a station at 16 miles -50 dBm doesn't come in at all /shrug