[evlatests] L-band RFI from VOIP phones.

[vdhawan at aoc.nrao.edu]

               L-band RFI from NEC DTerm VOIP phone.

Using the VLA in spectral line mode, we measured the 25MHz comb
from a phone placed temporarily in the vertex room of antenna 6.
A calibration tone at -70dBm was also recorded.

On the basis of Dan Mertely's lab measurements, the unshielded 
level of several spikes from 1300-1500MHz is -55+_5dBm, with spikes 
~5dB higher seen below 1GHz. To keep it simple we did just L band
1287-1513MHz, RCP, IF A, 6.25MHz BW, 50KHz resolution, 40sec
integration at each setting.

Bottom line first: 

The 25MHz harmonics are strong in the autocorrelation spectra of
antenna 6; absent with phone off; and absent in other antennas.

The measured levels, calibrated as below, imply a shielding from
vertex room interior to L-band (VLA) feed/receiver of ~70dB.

A further 75 dB is required to get the phone below the harmful
threshold of -200dBm at 1325-1500MHz (see footnote).

However, the phone emission may be variable. The 1300MHz emission 
in particular was 10-15dB stronger on the VLA than in the lab, and 
may demand 85dB shielding if confirmed.

A clean result from the calibration tone is that -70dBm radiated 
in the vertex room needs 60dB additional shielding.

There is much information in the cross-power spectra: leakage 
is seen into most other VLA antennas, where it correlates with 
antenna 6. I will give an update later.

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			Details:

Data were taken with the VOIP on and off; during the off sweep a
signal generator emitted 1343MHz at -70dBm as a calibration beacon 
in the vertex room, and leaked into the receiver at -141dBm. Ylva
Pihlstrom, EVLA Memo #47, Oct. 2002, got a near-identical result 
on antenna 22: -70dBm at 1440MHz radiated in the vertex room was 
received at -143dBm. 

Hence the loss from vertex room to receiver is 70dB.

Most of the 25MHz VOIP harmonics have about the same loss (L_V_Rx),
within their intrinsic variability.

F_MHz	 M_Rx		  Mert			L_V_Rx	   S_add

1300	-115 dBm	-53 -60 dBm		62 55	   85 dB 

1325	-129		-60 -68			69 61	   71

1343	-141 dBm	-70 dBm cal. tone	71 	   59 
			(ant 6, this meas.)

1350	-126		-54 -57			72 69	   74

1375	-133		-57			76	   67

1400	-128 voip on	-54 -64			74 64	   72

1400	-143 voip off.  ?

1425	no data		-62 -70			?	   68?

1440	-143 dBm	-70 dBm cal. tone	73 dB	   57 
			(ant 22, memo #47)
			
1450	-133		-60 -62			73 71	   67

1475	-143		-70 -73			73 70	   57

1500	-150		-70 -70			80 80	   50


M_Rx = Measured at VLA Rx output. The calibration is by comparing
	the line strength to T_sys in the band. The noise power, 
	i.e. the pedestal under the line, is P_sys = (k*T_sys*BW) 
	= -136dBm, for T_sys=30K, BW=50KHz. 
	A line as high as the pedestal in 1 channel is -136dBm.

	The weakest line detectable in autocorrelation was 
	0.05*P_sys or -150 dBm. 

Mert = Mertely measurement of VOIP, 2 runs in shielded room, 
	+20dB room correction factor applied.
 
	Also in this column are tones at -70dBm, 1343 and 1440MHz.

L_V_Rx = Inferred loss from vertex rm to Rx, comparing col 2, 3.

S_add  = Additional shielding to get column 2 below -200dBm.

----------------------
Footnote: The threshold is 10dB below the noise power in 5KHz =
1km/s at L-band, 8hr integration, 30K T_sys.  The gain of a 1m^2 
aperture at 20cm is 25 dB, so this is equivalent to the threshold 
of -203dBW/m^2 received in the isotropic sidelobes, as used in 
other definitions, e.g. EVLA memo#47.
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