Tuner input impedance varies over frequency. This has the effect of
non-linear Return Loss over the frequency range of RF energy arriving at the
input amplification device. This is caused by Phase Shift as parasitic LRC
at the input, the connector is a factor but the connector type whether F,
BNC, or MCX does not determine the characteristic input impedance of a tuner
or amplifier. The peaks and dips (usually 1 dip but sometimes 2) in the
Return Loss of a Tuner can vary as much as 30 dB or 1000 times, the dips are
usually very steep between 3 to 5 GHz.
Silicon transistors typically have a transition impedance from capacitive to
inductive around 300 MHz; series capacitive below 300 MHz, transitioning to
series inductive above 300 MHz (thus limiting its upper frequency
operation). Near 300 MHz, the impedance appears near resistive because the
phase shift at the gate or base of the silicon is near 0 at this frequency;
a matter of solid state physics.
For fixed frequency applications of tuners, to achieve maximum gain with
minimum noise, a matching network should be applied between the antennae and
the input of the tuner, preferably physically on the tuner input itself not
the antenna. Likewise, Matching Networks can be applied to the Antenna to
peak its gain but typically there is an amplifier between the antenna and
the tuner at the head end of the coaxial cable so peaking the antenna either
with a Matching Network or physically moving the relative orientation of the
(Driven) elements to peak the gain of the antenna is preferred. Antenna
impedance mismatch is a cause of loss of gain, so anything done to reduce
the parasitic loss of gain at the antenna reduces noise in the tuner. These
techniques are typically for applications of fixed frequency reception or
transmission.
Then there is reflection... this is why we have Balun Transformers.
Ideally, the tuner should be on the antenna with no feed coaxial cable; the
driven element of the antenna connected directly to the tuner's input. This
is why Tuner Dongles and single board SDRs are attractive, making it
possible (with difficulty) to place the tuner closer to the antennae. It is
now possible to wirelessly transfer the DATA from the tuner to the
Receiver/Computer with no physical wire connection between the tuner and the
receiver (SDR). IP/TCP-based DATA control/streaming is now a reality.
Lightning damage and Ground Loops are eliminated with wireless or fiber
communication.
Jay Salsburg
-------------------------
From:
http://www.radio-electronics.com/info/antennas/coax/rf-coaxial-cable-impedan
ce.php
Coax impedance / characteristic impedance
All feeders possess a characteristic impedance. For RF coax cable there are
two main standards that have been adopted over the years, namely 75 and 50
ohms.
75 ohm coax cable is used almost exclusively for domestic TV and VHF FM
applications. However for most commercial RF applications 50 ohms coax cable
has been taken as the standard for many years.
The reason for the choice of these two impedance standards is largely
historical but arises from the properties provided by the two impedance
levels:
75 ohm coax cable gives the minimum weight for a given loss
50 ohm coax cable gives the minimum loss for a given weight.
These two standards are used for the vast majority of coax cable which is
produced but it is still possible to obtain other impedances for specialist
applications. Higher values are often used for computer installations, but
other values including 25, 95 and 125 ohms are available. 25 ohm miniature
RF cable is extensively used in magnetic core broadband transformers. These
values and more are available through specialist coax cable suppliers.
-----Original Message-----
From: osmocom-sdr-***@lists.osmocom.org
[mailto:osmocom-sdr-***@lists.osmocom.org] On Behalf Of lists
Sent: Tuesday, August 26, 2014 1:35 AM
To: osmocom-***@lists.osmocom.org
Subject: Re: DVB-T receiver comparison
Other than frequency translation, AM is basically a linear process. So it
will show THD, noise, etc.
BTW, my DVB-T dongle has a MCX connector. I'm not really sure how serious
these manufacturers are in controlling impedance. Maybe they are 75 ohm, and
maybe not. There are 75 ohm BNCs, as determined by the center pin.
I have more audio than RF experience, but much of the testing is the same.
In audio, if you had your test source phased locked to the the A/D, you
SINAD can be computed direction from the FFT bins. Signal is one bin, noise
and distortion is everything else. [There is probably some RMS addition of
the bins, which you would have to look up.] Once the source and sampler are
not phased locked, then windowing comes into play and things get more
complicated.
It isn't all that different for your AM demodulated signal. In the case of
the DVB-T, they are not phased locked, so you will have to count a certain
number of bins as signals and the rest as noise.
For PSK or any type of digital demod, examining the eye or constellation
would make more sense. But I see nothing wrong with doing all these
evaluations with just AM, since it is a decent figure of merit test.
It has been my experience just using these dongles as NFM or FM radios, that
they are not particularly good at low frequencies (in the 100MHz range). I
only use them for 1090MHz mode-s.
On Tue, 26 Aug 2014 07:44:19 +0200
Post by friedtjI have just reproduced all measurements making sure to compensate for
frequency offset between the synthesizer and the local oscillators of
the DVB-T, and added a table summarizing the measurements (peak power
and noise floor of the RF and FM-demodulated signals).
Could you please comment on the FM demodulation sudden drop ? This is
indeed what I observe but I fail understanding the cause. I have
access to most modulation schemes: could you please advise on which
would be most relevant ? I have selected FM because it is the
modulation scheme I am aiming at in the case of NOAA POES reception,
although the 1500 MHz measurements are targeted towards GPS reception
so PSK would be more relevant indeed.
Thanks, JM
Post by David BasdenPost by friedtjIn order to quantify my abilty to receive weather satellite images
using DVB-T running with GNURadio, I have setup a very simple
* tune a frequency synthesizer to 137 MHz, FM modulation, 5 kHz
deviation and 2400 Hz FM frequency
* frequency synthesizer output power setup to -91 or -101 dBm
(same setting for all receivers, 2 sets of measurements)
* connect the output of the synthesizer to the input of the DVB-T
(in all cases, a new BNC connector was solder instead of the
original 75-ohm antenna connector),
* plot the RF signal FFT and the FFT of a WBFM demodulator block
as found in GNURadio (always same settings: 30 dB IF gain and 30
dB RF gain)
* assess the receiver quality by the signal to noise ratio of the FM output.
I'd suggest using something other than FM if you can find a good
reference. Looking at the output of an FM demod is going to mask
most of the differences you're looking for, and you're much more
likely to get a very sharp cutoff between having good output, and
not having an intelligible output at all.
David
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