dbm to volts (humax F2foxT)

I read in this review

http://www.stevelarkins.freeuk.com/humax_f2foxt.htm

that this box works down to 32 microvolts, which he calls ridiculously
low, but appears to be about -77dBm.

My GDB4 goes doem to -82dBm and is not sensitive enough.

Could any one confirm or otherwise, that the table here

http://www.moseleysb.com/mb/mv2dbm.html

and the rule of thumb he

http://mrtmag.com/mag/radio_using_converting_rf/:

Conversely, to convert dBm to microvolts follow this example:

Convert -110dBm to microvolts. Start with 0dBm = 0.25V, or 250,000µV.
Now, for each -20dB, move the decimal point one place to the left. Moving
five places to the left, we get 2.5µV. Now, six more decibels would yield
a level of 1.25µV. Another 4dB reduction would be a multiplication factor
of 1 ÷ 1.6, or 0.625, to yield:

both indicate that the GDB4 is much better at about 19 microvolts.

Or am I missing something, and should I buy a humax, as I'm greatly
tempted

Any help appreciated

mike

mike wrote:
I read in this review

http://www.stevelarkins.freeuk.com/humax_f2foxt.htm

that this box works down to 32 microvolts, which he calls ridiculously
low, but appears to be about -77dBm.

My GDB4 goes doem to -82dBm and is not sensitive enough.

Could any one confirm or otherwise, that the table here

http://www.moseleysb.com/mb/mv2dbm.html

and the rule of thumb he

http://mrtmag.com/mag/radio_using_converting_rf/:
Both look ok at a glance.

If you understand dBm you probably don't need advice here.

Steve

"Steve" wrote in message
...
mike wrote:
I read in this review

http://www.stevelarkins.freeuk.com/humax_f2foxt.htm

that this box works down to 32 microvolts, which he calls
ridiculously
low, but appears to be about -77dBm.

My GDB4 goes doem to -82dBm and is not sensitive enough.

Could any one confirm or otherwise, that the table here

http://www.moseleysb.com/mb/mv2dbm.html

and the rule of thumb he

http://mrtmag.com/mag/radio_using_converting_rf/:
Both look ok at a glance.

If you understand dBm you probably don't need advice here.

Steve

OP - do you have vision problems? The review says 32dBuV which is 32dB
above a microvolt - or in old money 40uV.

The table in the second link relating dBuV/dBm/power is at 50R. As
0dBm is a power of 1mW then by Ohms Law the voltage that 0dBm will
portend depends upon the load resistance (more correctly impedence.)
TV in the UK is 75R.

TV in the UK is/was generally engineered to provide, on average, 1mV
of signal to most households, although in fringe areas a minimum
signal of 250uV was deemed acceptable. As DTTV currently transmits
20dB below analogue TV peak then the minimum signal that most people
should see is 25uV. Following the maths above -82dBm would be around
23uV. If the GDB4 won't work then you should consider that you are not
served by DTTV.

--
Woody

harrogate3 at ntlworld dot com

OP - do you have vision problems?

No, just fundamental understanding problems
The review says 32dBuV which is 32dB

Whoops, it appears I _do_ have vision problems!

above a microvolt - or in old money 40uV.


TV in the UK is/was generally engineered to provide, on average, 1mV
of signal to most households, although in fringe areas a minimum
signal of 250uV was deemed acceptable. As DTTV currently transmits
20dB below analogue TV peak then the minimum signal that most people
should see is 25uV. Following the maths above -82dBm would be around
23uV. If the GDB4 won't work then you should consider that you are not
served by DTTV.

SO, I think your conclusion is, but it would be nice to be sure, the
GDB4, which works pretty well most of the tine on most muxes, is more
sensitive than the Humax; or am I missing any other factors like error
correction, etc.

Sorry to be importunate, but analogue round here varies from fine to
unusable, even though I can choose CP or Sudbury, and digital only has
the two extremes.

I've got sky free to air, but I'd love to cut down the number of damn
boxes I must use.

mike

"harrogate3" wrote in message
...
TV in the UK is/was generally engineered to provide, on average, 1mV
of signal to most households, although in fringe areas a minimum
signal of 250uV was deemed acceptable. As DTTV currently transmits
20dB below analogue TV peak then the minimum signal that most people
should see is 25uV. Following the maths above -82dBm would be around
23uV. If the GDB4 won't work then you should consider that you are not
served by DTTV.

I don't think the absolute sensitivity of a receiver is all that crucial.
When DTT signal levels are on the low side the problems that arise are
caused by interference of one sort or another which effectively raises the
threshold of acceptable reception well above the receiver's theoretical
ability to decode a weak signal. I recently had a problem in which a Fox 2T
was struggling with two muxes. The signal levels from the aerial (before
amplification) were marginal, but the results seemed much worse than a
simple measure of signal level would suggest. On site measurement of c/n
ratio said that general background noise (no specific source) was high. The
aerial was on a city centre building. To prove the point to myself I took
the receiver to the workshop and fed it with a completely clean signal which
I attenuated to the point where the relevant muxes gave up the ghost. The
signal level presented to the receiver was then 10dB lower than that on
site. To me this shows that noise generated inside the receiver or noise
picked up by the receiver directly is not very significant. However, noise
picked up on a poorly screened flylead or wallplate can be very significant
indeed. I've seen many an instance in which the TV set itself has affected
DTT reception on the set-top box. Good flyleads provide the solution. Once
though, the scart connection was the culprit!

I've often noticed that DTT reception in my motorhome, when parked out in
the wilds, is surprisingly good. But if I turn on the rear view LCD screen
or the digital volt/amp meters the reception can fail.

When I encounter a 'mysterious' case of poor DTT reception the cause is
usually co-channel interference.

Bill

"Bill Wright" wrote in message
...

"harrogate3" wrote in message
...
TV in the UK is/was generally engineered to provide, on average,
1mV
of signal to most households, although in fringe areas a minimum
signal of 250uV was deemed acceptable. As DTTV currently transmits
20dB below analogue TV peak then the minimum signal that most
people
should see is 25uV. Following the maths above -82dBm would be
around
23uV. If the GDB4 won't work then you should consider that you are
not
served by DTTV.

I don't think the absolute sensitivity of a receiver is all that
crucial.
When DTT signal levels are on the low side the problems that arise
are
caused by interference of one sort or another which effectively
raises the
threshold of acceptable reception well above the receiver's
theoretical
ability to decode a weak signal. I recently had a problem in which a
Fox 2T
was struggling with two muxes. The signal levels from the aerial
(before
amplification) were marginal, but the results seemed much worse than
a
simple measure of signal level would suggest. On site measurement of
c/n
ratio said that general background noise (no specific source) was
high. The
aerial was on a city centre building. To prove the point to myself I
took
the receiver to the workshop and fed it with a completely clean
signal which
I attenuated to the point where the relevant muxes gave up the
ghost. The
signal level presented to the receiver was then 10dB lower than that
on
site. To me this shows that noise generated inside the receiver or
noise
picked up by the receiver directly is not very significant. However,
noise
picked up on a poorly screened flylead or wallplate can be very
significant
indeed. I've seen many an instance in which the TV set itself has
affected
DTT reception on the set-top box. Good flyleads provide the
solution. Once
though, the scart connection was the culprit!

I've often noticed that DTT reception in my motorhome, when parked
out in
the wilds, is surprisingly good. But if I turn on the rear view LCD
screen
or the digital volt/amp meters the reception can fail.

When I encounter a 'mysterious' case of poor DTT reception the cause
is
usually co-channel interference.

Bill



Thanks Bill, saved me wearing out any more of my finger ends!


--
Woody

harrogate3 at ntlworld dot com

I don't think the absolute sensitivity of a receiver is all that
crucial. When DTT signal levels are on the low side the problems that
arise are caused by interference of one sort or another which
effectively raises the threshold of acceptable reception well above
the receiver's theoretical ability to decode a weak signal.
snip
To me this shows that noise generated inside the
receiver or noise picked up by the receiver directly is not very
significant. However, noise picked up on a poorly screened flylead or
wallplate can be very significant indeed. I've seen many an instance
in which the TV set itself has affected DTT reception on the set-top
box. Good flyleads provide the solution. Once though, the scart
connection was the culprit!


When I encounter a 'mysterious' case of poor DTT reception the cause
is usually co-channel interference.

Thanks, Bill, it sort of confirms the GDB4 is about as good as I'll get.

The problem is I'm just the wrong side of the brow of a big hill between
me and CP; I don't think I can get any more height on the aerial on my
bungerlow, (it would rip the chimbley off) the hill is crested by loadsa
trees, and I'm subject to very bad interfernce from our continental
partners??? in certain atmospheric conditions.

And I don't expect to last to 2012 to see an improvement.

But at least I can put the "new box" idea to bed again - it comes up
every time I see a good review; I'll check the leads out, I've got some
good ones and some others, and the flyleads I'm not at all sure of - they
generally came with bits of kit, but don't inspire confidence.

I looked on Maplin's - they sell some but at 4 squid each they look just
like the stuff that I've already got; do you know where I could get some
good ones?

mike

On Aug 3, 10:32 am, mike wrote:

I looked on Maplin's - they sell some but at 4 squid each they look just
like the stuff that I've already got; do you know where I could get some
good ones?

The best thing is to make your own, using lengths of CT-100.

Particularly for

I looked on Maplin's - they sell some but at 4 squid each they look just
like the stuff that I've already got; do you know where I could get some
good ones?

Are you able to make some yourself with double screened co-ax (CT100, PF100
etc) and good quality plugs soldered on?

I've yet to beat home made ones, certainly for price vs quality.

On Aug 3, 10:32 am, mike wrote:

I looked on Maplin's - they sell some but at 4 squid each they look just
like the stuff that I've already got; do you know where I could get some
good ones?

Best thing is to make your own, using lengths of CT-100, and soldered
joints on the Belling Lee co-ax plugs. Better stil, use F-type
connectors, and F-type-BL adaptors where needed. The problem with
most ready made flyleads is they're very lossy, particularly at the
top end of UHF (exactly where Sudbury's DTT channels are).

(Sorry about previous incomplete post, finger trouble !)