diff --git a/src/ophyd_async/core/mock_signal_backend.py b/src/ophyd_async/core/mock_signal_backend.py
index 77f5fdc2ae..4c0e94171f 100644
--- a/src/ophyd_async/core/mock_signal_backend.py
+++ b/src/ophyd_async/core/mock_signal_backend.py
@@ -10,7 +10,7 @@
 from ophyd_async.core.utils import DEFAULT_TIMEOUT, ReadingValueCallback, T
 
 
-class MockSignalBackend(SignalBackend):
+class MockSignalBackend(SignalBackend[T]):
     def __init__(
         self,
         datatype: Optional[Type[T]] = None,
@@ -31,11 +31,11 @@ def __init__(
 
         if not isinstance(self.initial_backend, SoftSignalBackend):
             # If the backend is a hard signal backend, or not provided,
-            # then we create a soft signal to mimick it
+            # then we create a soft signal to mimic it
 
             self.soft_backend = SoftSignalBackend(datatype=datatype)
         else:
-            self.soft_backend = initial_backend
+            self.soft_backend = self.initial_backend
 
     def source(self, name: str) -> str:
         if self.initial_backend:
@@ -65,9 +65,6 @@ async def put(self, value: Optional[T], wait=True, timeout=None):
     def set_value(self, value: T):
         self.soft_backend.set_value(value)
 
-    async def get_descriptor(self, source: str) -> Descriptor:
-        return await self.soft_backend.get_descriptor(source)
-
     async def get_reading(self) -> Reading:
         return await self.soft_backend.get_reading()
 
diff --git a/src/ophyd_async/core/signal_backend.py b/src/ophyd_async/core/signal_backend.py
index 7b04ab1152..ef8e983421 100644
--- a/src/ophyd_async/core/signal_backend.py
+++ b/src/ophyd_async/core/signal_backend.py
@@ -14,7 +14,7 @@ class SignalBackend(Generic[T]):
 
     #: Like ca://PV_PREFIX:SIGNAL
     @abstractmethod
-    def source(name: str) -> str:
+    def source(self, name: str) -> str:
         """Return source of signal. Signals may pass a name to the backend, which can be
         used or discarded."""