Portsdown NF Meter - Revision history

G4FRE: /* Lime/Pluto Gain */

2025-10-01T15:37:23Z

Lime/Pluto Gain

G4FRE: /* Sources of Error */

2025-10-01T15:02:20Z

Sources of Error

G8GKQ at 16:17, 27 April 2024

2024-04-27T16:17:02Z

G8GKQ at 17:02, 10 December 2021

2021-12-10T17:02:12Z

G8GKQ: /* Noise head PSU */

2021-12-03T18:15:33Z

Noise head PSU

G8GKQ: /* Noise head PSU */

2021-12-03T18:07:41Z

Noise head PSU

G8GTZ: /* Making a Simple Measurement */

2021-11-24T12:51:07Z

Making a Simple Measurement

G8GTZ: /* Preparing for a Measurement */

2021-11-24T12:50:54Z

Preparing for a Measurement

G8GTZ: /* Equipment Required */

2021-11-24T12:50:43Z

Equipment Required

G8GTZ: /* General Principle */

2021-11-24T12:50:34Z

General Principle

@@ Line 69: / Line 69: @@
 ====Lime/Pluto Gain====
-Now, the gain of the LimeSDR or Pluto needs to be adjusted so that it is operating in its linear region.  Use the "Gain Up" and "Gain Down" buttons, so that the "Noise Off" level shown at the bottom left of the screen is between -65 dB and -55 dB (LimeSDR) or between -52 dB and -42 dB (Pluto).  If set too low, the indication will change colour and a message advising an increase in Lime/Pluto Gain will be displayed.  If the gain is set too high, there is a risk that high gain preamps may overload the system when tested, leading to inaccurate readings.
+Now, the gain of the LimeSDR or Pluto needs to be adjusted so that it is operating in its linear region.  With the noise source connected directly to the Lime SDR, use the "Gain Up" and "Gain Down" buttons, so that the "Noise Off" level shown at the bottom left of the screen is between -65 dB and -55 dB (LimeSDR) or between -52 dB and -42 dB (Pluto).  If set too low, the indication will change colour and a message advising an increase in Lime/Pluto Gain will be displayed.  If the gain is set too high, there is a risk that high gain preamps may overload the system when tested, leading to inaccurate readings.
 The Meter will now display the Noise Figure of your LimeSDR.  This is very dependent on screening and digital noise pickup, but should be in the range 5 dB to 10 dB between 146 and 1255 MHz.

@@ Line 117: / Line 117: @@
 SDR non-linearity
-Temperature effects
+Temperature effects on SDR
 ===References:===

@@ Line 1: / Line 1: @@
-The Portsdown 4 includes a Noise Figure Meter that uses the LimeSDR Mini and a user-provided noise source and custom switching power supply to provide an indication of the performance of preamps and transverters.  It is hoped to describe this emerging capability in CQ-TV 274 (Winter 2021).  This Wiki page is purely to inform the early beta testers before the capability goes on general release.
+The Portsdown 4 includes a Noise Figure Meter that uses the LimeSDR Mini or a Pluto SDR with a user-provided noise source and custom switching power supply to provide an indication of the performance of preamps and transverters.  This capability was described in CQ-TV 274 (Winter 2021).
 ==General Principle==
@@ Line 13: / Line 13: @@
 [[File:NF Block Diagram.jpg|800px]]
-To measure noise figure using the Portsdown 4, a Lime SDR Mini, calibrated noise source and matching switched power supply are required.
+To measure noise figure using the Portsdown 4, a Lime SDR Mini (or a Pluto SDR), calibrated noise source and matching switched power supply are required.
 ====Noise heads====
@@ Line 61: / Line 61: @@
 ====Measurement bandwidth====
-It is suggested that the measurement bandwidth is left at 2 MHz (as was standard on the early HP Noise Figure meters) but it can be adjusted down to 500 kHz and up to 20 MHz at this point.
+It is suggested that the measurement bandwidth is left at 2 MHz (as was standard on the early HP Noise Figure meters) but it can be adjusted down to 500 kHz (LimeSDR only - the Pluto is limited to 2 MHz) and up to 20 MHz at this point.
 ====ENR====
@@ Line 67: / Line 67: @@
 Next, you need to enter the ENR of your noise source.  Touch the ENR button and enter the exact ENR for your noise source at the selected frequency (read from its calibration chart).
-====Lime Gain====
+====Lime/Pluto Gain====
-Now, the gain of the LimeSDR needs to be adjusted so that it is operating in its linear region.  Use the "Gain Up" and "Gain Down" buttons, so that the "Noise Off" level shown at the bottom left of the screen is between -65 dB and -55 dB.  If set too low, the indication will change colour and a message advising an increase in Lime Gain will be displayed.  If the gain is set too high, there is a risk that high gain preamps may overload the system when tested, leading to inaccurate readings.
+Now, the gain of the LimeSDR or Pluto needs to be adjusted so that it is operating in its linear region.  Use the "Gain Up" and "Gain Down" buttons, so that the "Noise Off" level shown at the bottom left of the screen is between -65 dB and -55 dB (LimeSDR) or between -52 dB and -42 dB (Pluto).  If set too low, the indication will change colour and a message advising an increase in Lime/Pluto Gain will be displayed.  If the gain is set too high, there is a risk that high gain preamps may overload the system when tested, leading to inaccurate readings.
 The Meter will now display the Noise Figure of your LimeSDR.  This is very dependent on screening and digital noise pickup, but should be in the range 5 dB to 10 dB between 146 and 1255 MHz.
@@ Line 81: / Line 81: @@
 ===Measuring Gain and NF of a Preamp===
-By calibrating the noise levels of the LimeSDR, it is then possible to calculate the Gain and NF of a preamp and exclude the NF contribution of the LimeSDR Mini.  This procedure only works for one frequency and one Lime Gain setting, and needs to be repeated if frequency, Lime Gain or Bandwidth are altered.
+By calibrating the noise levels of the LimeSDR, it is then possible to calculate the Gain and NF of a preamp and exclude the NF contribution of the LimeSDR Mini or Pluto.  This procedure only works for one frequency and one Gain setting, and needs to be repeated if frequency, Gain or Bandwidth are altered.
 Note that this calibration procedure is not the same as calibrating the LimeSDR; it is calibrating the noise response of the LimeSDR.
@@ Line 89: / Line 89: @@
 [[File:nfdevice.jpg|400px]]
-Once calibrated, the Lime Gain buttons are locked (as adjusting them would invalidate the calibration).  To adjust the Lime Gain, press the "Calibrated" button again to come out of Calibrated mode.
+Once calibrated, the Gain buttons are locked (as adjusting them would invalidate the calibration).  To adjust the Gain, press the "Calibrated" button again to come out of Calibrated mode.
 ===Measuring a Transverter or Receive Converter===

← Older revision		Revision as of 17:02, 10 December 2021
Line 1:		Line 1:
−	The Portsdown 4 includes a Noise Figure Meter ~~(STILL UNDER DEVELOPMENT)~~ that uses the LimeSDR Mini and a user-provided noise source and custom switching power supply to provide an indication of the performance of preamps and transverters. It is hoped to describe this emerging capability in CQ-TV 274 (Winter 2021). This Wiki page is purely to inform the early beta testers before the capability goes on general release.	+	The Portsdown 4 includes a Noise Figure Meter that uses the LimeSDR Mini and a user-provided noise source and custom switching power supply to provide an indication of the performance of preamps and transverters. It is hoped to describe this emerging capability in CQ-TV 274 (Winter 2021). This Wiki page is purely to inform the early beta testers before the capability goes on general release.

	==General Principle==		==General Principle==

@@ Line 37: / Line 37: @@
 The drive for the noise source switching comes from pin 26 of the Portsdown GPIO port, and is at 3.3v when the noise source is on, and 0v when the noise source is off.
-BATC are producing a PCB which will be in the member's shop.  The design is described here [[Noise Source PSU]].  The PCB schematic is here [[:media:NoiseHeadPSU.pdf]]
+BATC are producing a PCB which will be in the member's shop.  The design is described here [[Noise Source PSU]].
 ==Preparing for a Measurement==

@@ Line 73: / Line 73: @@
 The Meter will now display the Noise Figure of your LimeSDR.  This is very dependent on screening and digital noise pickup, but should be in the range 5 dB to 10 dB between 146 and 1255 MHz.
-===Making a Simple Measurement===
+==Making a Simple Measurement==
 If you now insert your preamp or transverter (the Device Under Test)  between the noise source and the LimeSDR, you should see the "System" NF improve (hopefully) and you can make adjustments to your device to optimise its performance.

@@ Line 39: / Line 39: @@
 BATC are producing a PCB which will be in the member's shop.  The PCB schematic is here [[:media:NoiseHeadPSU.pdf]]
-===Preparing for a Measurement===
+==Preparing for a Measurement==
 Ensure you have the latest Portsdown software loaded (Version 202110300 published Nov 2nd 2021 or later) and connect your LimeSDR to the Portsdown by USB.  Note - the LimeSDR should have gateware V1.30 or the LimeDVB gateware loaded.

@@ Line 9: / Line 9: @@
 The noise contribution from amateur preamps and transverters is normally stated as a logarithmic "Noise Figure" measured in dB.  Levels around 3 dB are acceptable for VHF receivers (where there is a lot of atmospheric noise) whereas good microwave receivers typically achieve between 1 and 2 dB.  Lower noise figures are desirable for EME and satellite use where the background noise is much lower.  Lower noise figures may also be stated as a noise temperature.  There is a direct logarithmic relationship betwen the two.  This Wiki page provides a full explanation: https://en.wikipedia.org/wiki/Noise_temperature .
-===Equipment Required===
+==Equipment Required==
 [[File:NF Block Diagram.jpg|800px]]

@@ Line 1: / Line 1: @@
 The Portsdown 4 includes a Noise Figure Meter (STILL UNDER DEVELOPMENT) that uses the LimeSDR Mini and a user-provided noise source and custom switching power supply to provide an indication of the performance of preamps and transverters.  It is hoped to describe this emerging capability in CQ-TV 274 (Winter 2021).  This Wiki page is purely to inform the early beta testers before the capability goes on general release.
-===General Principle===
+==General Principle==
 The limiting factor on the performance of many microwave receivers and some VHF receivers is the amount of noise generated by the input stage of the receiver.  G8GTZ discussed this in his excellent [https://www.youtube.com/watch?v=lfSi7vTQK44 CAT 21 Video] which is essential viewing before trying to use the Portsdown 4 Noise Figure Meter.