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New CODD Pick-up Amplifier |
The input-referred noise level of the amplifiers is measured using the Y-method. The table below enumerates the initial measured Y factors and noise levels for the Σ amplifier, which has three FETs, an input matching section, but no low-pass filter and attenuator relays.
| Frequency | Y [dB] | Un [V/√Hz] |
|---|---|---|
| 500k | 3.4 | 270p |
| 1M | 3.5 | 260p |
| 2M | 3.5 | 260p |
| 5M | 3.2 | 290p |
| 10M | 3.1 | 300p |
| 20M | 2.7 | 350p |
| 50M | 1.2 | 650p |
The corresponding values for the Y amplifier, which has two FETs, a low-pass filter and all attenuator relays installed are:
| Frequency | Y [dB] | Un [V/√Hz] |
|---|---|---|
| 500k | 3.0 | 310p |
| 1M | 2.6 | 360p |
| 2M | 2.8 | 340p |
| 5M | 2.7 | 350p |
| 10M | 2.1 | 400p |
| 20M | 1.8 | 490p |
| 50M | 0.1 | 2.5n |
The differences between the two are believed to be mainly due to the different number of FETs in the input stage. The values nicely match the calculated noise levels of the amplifiers. The main contributor is the voltage noise of the input FETs, two or three Philips BF862 JFETs in parallel. The voltage noise of a single FET is 0.8nV/√Hz. Three in parallel reduce that by a factor of √3 to 460pV/√Hz. Then the input transformer reduces this by another factor of two, to 230pV/√Hz. The slightly higher value of 260pV/√Hz is due to other contributors in the circuitry.
In January 2007, after one year in the ring, the noise of a subset of the installed
amplifiers was measured with the following results:
At 2MHz:
| Chassis nr | Ch | Y[dB] | Vn |
|---|---|---|---|
| 0x1e | S | 3.1 | 300p |
| 0x1e | Y | 3.1 | 300p |
| 0x1a | X | 3.1 | 300p |
| 0x1a | S | 3.2 | 290p |
| 0x1a | Y | 3.3 | 280p |
| 0x22 | X | 3.3 | 280p |
| 0x22 | S | 3.3 | 280p |
| 0x22 | Y | 3.4 | 270p |
| proto | X | 3.0 | 310p |
| proto | S | 3.3 | 280p |
| proto | Y | 3.3 | 280p |
| 0x13 | X | 3.3 | 280p |
| 0x13 | S | 3.3 | 280p |
| 0x13 | Y | 3.3 | 280p |
| 0x2d | X | 3.2 | 290p |
| 0x2d | S | 3.3 | 280p |
| 0x2d | Y | 3.3 | 280p |
| 0x38 | X | 3.3 | 280p |
| 0x38 | S | 3.3 | 280p |
| 0x38 | Y | 3.3 | 280p |
| 0x06 | X | 3.3 | 280p |
| 0x06 | S | 3.2 | 290p |
| 0x06 | Y | 3.2 | 290p |
| 0x3d | X | 3.5 | 260p |
| 0x3d | S | 3.5 | 260p |
| 0x3d | Y | 3.4 | 270p |
| 0x37 | X | 3.3 | 280p |
| 0x37 | S | 3.4 | 270p |
| 0x37 | Y | 3.4 | 270p |
| 0x2c | X | 3.4 | 270p |
| 0x2c | S | 3.2 | 290p |
| 0x2c | Y | 3.4 | 270p |
| 0x3f | X | 3.5 | 260p |
| 0x3f | S | 3.4 | 270p |
| 0x3f | Y | 3.4 | 270p |
| 0x36 | X | 3.3 | 280p |
| 0x36 | S | 3.3 | 280p |
| 0x36 | Y | 3.4 | 270p |
| 0x1c | X | 3.4 | 270p |
| 0x1c | S | 3.4 | 270p |
| 0x1c | Y | 3.3 | 280p |
On chassis 0x1c:
| Frequency | Y [dB] | Un [V/√Hz] |
|---|---|---|
| 500k | 2.7 | 350p |
| 1M | 3.2 | 290p |
| 2M | 3.3 | 280p |
| 5M | 3.2 | 290p |
| 10M | 2.8 | 340p |
| 20M | 2.8 | 400p |
| 50M | 0.6 | 970p |